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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C.   20549

FORM 11-K

(Mark One)

x                              ANNUAL REPORT PURSUANT TO SECTION 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

For the fiscal year ended December 31, 2012

OR

o                                 TRANSITION REPORT PURSUANT TO SECTION 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

For the transition period from       to

Commission file number 1-09328

A.                        Full title of the plan and the address of the plan, if different from that of the issuer named below:

ECOLAB PUERTO RICO SAVINGS PLAN

B.                        Name of issuer of the securities held pursuant to the plan and the address of its principal executive office:

ECOLAB INC.

370 Wabasha Street North

Saint Paul, Minnesota  55102-1390

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Ecolab Puerto Rico Savings Plan

REPORT OF FINANCIAL STATEMENTS

As of December 31, 2012 and 2011

and

for the year ended December 31, 2012 and the period from December 12, 2011 to December 31, 2011

AND SUPPLEMENTAL SCHEDULES

as of December 31, 2012 and 2011

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INDEX

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Report of Independent Registered Public Accounting Firm

To the Plan Administrator and Trustees

Ecolab Puerto Rico Savings Plan

We have audited the accompanying statements of net assets available for benefits of the Ecolab Puerto Rico Savings Plan (the Plan) as of December 31, 2012 and 2011, and the related statements of changes in net assets available for benefits for the year ended December 31, 2012 and the period from December 12, 2011 to December 31, 2011.  These financial statements are the responsibility of the Plan’s management.  Our responsibility is to express an opinion on these financial statements based on our audits.

We conducted our audits in accordance with the standards of the Public Company Accounting Oversight Board (United States).  Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement.  An audit includes examining, on a test basis, evidence supporting the amounts and disclosures in the financial statements.  An audit also includes assessing the accounting principles used and significant estimates made by management, as well as evaluating the overall financial statement presentation.  We believe that our audits provide a reasonable basis for our opinion.

As discussed in Note 1 to the financial statements, effective December 12, 2011 the Plan Sponsor created this Plan for certain individuals employed by Ecolab Inc.’s wholly owned subsidiary, Ecolab Manufacturing Inc.  Prior to this date, these individuals were included in the Ecolab Savings Plan and ESOP.

In our opinion, the financial statements referred to above present fairly, in all material respects, the net assets available for benefits of the Plan as of December 31, 2012 and 2011, and the changes in net assets available for benefits for the year ended December 31, 2012 and for the period from December 12, 2011 to December 31, 2011, in conformity with accounting principles generally accepted in the United States of America.

Our audits were performed for the purpose of forming an opinion on the basic financial statements taken as a whole. The supplemental schedules of Schedule H, Line 4i — Schedule of Assets (Held at End of Year) as of December 31, 2012 and 2011, is presented for the purpose of additional analysis and is not a required part of the basic financial statements but is supplementary information required by the Department of Labor’s Rules and Regulations for Reporting and Disclosure under the Employee Retirement Income Security Act of 1974.  These supplemental schedules are the responsibility of the Plan’s management.  The supplemental schedules have been subjected to the auditing procedures applied in the audits of the basic financial statements and, in our opinion, are fairly stated in all material respects in relation to the basic financial statements taken as a whole.

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ECOLAB PUERTO RICO SAVINGS PLAN

STATEMENT OF NET ASSETS AVAILABLE FOR BENEFITS

December 31, 2012 and 2011

The accompanying notes are an integral

part of the financial statements.

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ECOLAB PUERTO RICO SAVINGS PLAN

STATEMENT OF CHANGES IN NET ASSETS AVAILABLE FOR BENEFITS

for the year ended December 31, 2012 and

for the period from December 12, 2011 to December 31, 2011

The accompanying notes are an integral

part of the financial statements

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ECOLAB PUERTO RICO SAVINGS PLAN

NOTES TO FINANCIAL STATEMENTS

1.                          Description of Plan:

The following brief description of the Ecolab Puerto Rico Savings Plan (“the Plan”) is provided for general information purposes only.  Participants should refer to the Plan document for more complete information regarding the Plan’s definitions, benefits, eligibility and other matters.

TRANSFERS:

Effective December 12, 2011, this plan was formed as a new plan and trust.  The Plan is for the benefit of certain individuals employed by Ecolab Inc.’s wholly owned subsidiary, Ecolab Manufacturing Inc. (“the Company”), and is subject to income taxation under Puerto Rico laws. The Ecolab Savings Plan and ESOP was amended to discontinue participation in such plan of employees eligible to participate in the new Plan and as such, the benefits, assets and liabilities of such participants were transferred to the trust of the Plan.

GENERAL AND ELIGIBILITY:

The Plan is a contributory qualified defined contribution plan available to certain employees of the Company in Puerto Rico. Employees who are employed by the Company and who are subject to income taxation under the laws of Puerto Rico may participate in the Plan as of the first day of the month on or after their date of hire, provided they are not subject to a collective bargaining agreement which does not provide for their participation in the Plan. Employee participation in the Plan is voluntary.

The Plan is subject to the provisions of the Employee Retirement Income Security Act of 1974, as amended (“ERISA”) and the Puerto Rico tax code (“the Code”). The Plan and trust are located and qualified only in Puerto Rico.

CONTRIBUTIONS:

Contributions are made to the Plan as “before-tax savings contributions” and “employer matching contributions.”

Before-tax savings contributions are contributions made by the Company on behalf of participants who have agreed to have their taxable compensation reduced. Participants may reduce their compensation up to 25% (subject to a Puerto Rico statutory annual maximum of $13,000 for 2012) for the purpose of making before-tax savings contributions to the Plan.

Participants who have attained age 50 or above are allowed to make additional catch-up contributions in accordance with Puerto Rico laws (up to $1,500 in 2012).

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ECOLAB PUERTO RICO SAVINGS PLAN

NOTES TO FINANCIAL STATEMENTS

1.                          Description of Plan (continued):

Participant contributions of up to 3% of eligible compensation are matched 100% by the Company and participant contributions over 3% and up to 5% of eligible compensation are matched 50% by the Company. The Plan also allows additional employer matching contributions to true-up the employer match. This true-up ensures all participants receive their full annualized employer match.

The levels of contributions made by or on behalf of participants who are highly compensated, as defined in the Code, are subject to limitations under the Code.

VESTING:

Participants are fully vested in their account at all times.

PLAN BENEFITS:

As participants are fully vested at all times, benefits to participants are equal to their account balances.  Upon retirement, death, disability or separation from service, a distribution may be made to the participant or beneficiary equal to the participant’s account balance. Employees may be eligible to withdraw part or all of their account balance upon attainment of age 59 1/2. Loans and in-service withdrawals for hardships are also available. A participant distribution or withdrawal from the Plan generally is subject to taxation under the tax rules applicable to Puerto Rico residents.

NOTES RECEIVABLE FROM PARTICIPANTS:

Active participants (and beneficiaries who are parties in interest as defined by ERISA) are permitted to borrow from their accounts.  The total amount of a participant’s note may not exceed the lesser of (a) $50,000 minus the participant’s highest outstanding note balance for the previous twelve-month period, or (b) 50% of the participant’s interest in his or her account. When a note is granted, the appropriate account balances are reduced and a separate note account is created.  Note payments, together with interest at a market rate determined by the Plan Administrator, are repaid generally over 5 years unless the note is for the purchase of a principal residence, in which case the term can be up to 10 years.  Notes receivable from participants at December 31, 2012 had interest rates ranging from 3.25% to 5.00% and are due at various dates through June 2017. A participant can have no more than two notes outstanding at any time. Notes receivable from participants are collateralized by the borrower’s account balance and are repaid through ratable payroll deductions.

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ECOLAB PUERTO RICO SAVINGS PLAN

NOTES TO FINANCIAL STATEMENTS

1.                          Description of Plan (continued):

PARTICIPANT ACCOUNTS AND ALLOCATION:

Banco Popular (de Puerto Rico) (“Banco Popular”) provides investment management, recordkeeping and trustee services for the Plan directly or indirectly through one or more of its subsidiaries.  The trust agreement authorizes services to be performed by the trustee, its agents or affiliates.

Each participant’s account is credited with the participant’s contributions, the employer matching contributions and investment income thereon, net of Plan expenses. The benefit to which a participant is entitled is the benefit that can be provided from the participant’s vested account.

All participant contributions made under the Plan are paid to and invested by Banco Popular in one or more of the available investment options as directed by the participants.

Participants are allowed to allocate their entire account balance in any combination of the available investment options.

PLAN TERMINATION:

Although it has not expressed any intent to do so, the Company has the right under the Plan to discontinue its contributions at any time and to terminate the Plan subject to the provisions of ERISA.

2.                          Summary of Significant Accounting Policies:

BASIS OF PRESENTATION:

The accompanying financial statements have been prepared on the accrual basis of accounting.

Investment contracts held by a defined contribution plan are required to be reported at fair value. However, contract value is the relevant measurement attribute for that portion of the net assets available for benefits of a defined contribution plan attributable to fully benefit-responsive investment contracts because contract value is the amount participants would receive if they were to initiate permitted transactions under the terms of the Plan. The statement of net assets available for benefits presents the fair value of the investment contracts as well as the adjustment of the fully benefit-responsive investment contracts from fair value to contract value; no such adjustment was applicable for 2012. The statement of changes in net assets available for benefits is prepared using the contract value basis for fully benefit-responsive investment contracts.

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ECOLAB PUERTO RICO SAVINGS PLAN

NOTES TO FINANCIAL STATEMENTS

2.                          Summary of Significant Accounting Policies (continued):

USE OF ESTIMATES:

The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, and changes therein, and disclosure of contingent assets and liabilities. Actual results could differ from those estimates.

VALUATION OF INVESTMENTS AND INCOME RECOGNITION:

Banco Popular holds the Plan’s assets and executes transactions therein based upon instructions received from the Plan Administrator, Ecolab Inc.  and the participants of the Plan.  The Plan’s investments are stated at fair value. Fair value is the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. See Note 4 for discussion of fair value measurements.

Interest income is recorded as earned on an accrual basis and dividend income is recorded on the ex-dividend date.  Purchases and sales of securities and realized gains and losses related to sales of investments are recorded on a trade-date basis. Unrealized gains and losses are recorded based on the fair values as of the reporting date.

NOTES RECEIVABLE FROM PARTICIPANTS:

Notes receivable from participants are measured at their unpaid principal balances plus any accrued interest. Interest income is recorded on the accrual basis.  Related fees are recorded as administrative expenses and are expensed when they are incurred.  No allowance for credit losses has been recorded as of December 31, 2012 or 2011.  If a participant ceases to make loan repayments and the Plan Administrator deems the participant loan to be in default, the participant loan balance is reduced and a benefit payment is recorded.

CONTRIBUTIONS:

Participant before-tax contributions are recorded in the period the employer makes the payroll deductions. Employer matching contributions are recorded based on participant contributions in the same period.

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ECOLAB PUERTO RICO SAVINGS PLAN

NOTES TO FINANCIAL STATEMENTS

2.                          Summary of Significant Accounting Policies (continued):

RISKS AND UNCERTAINTIES:

The Plan provides for various investment options in various combinations of investment funds.  Investments are exposed to various risks, such as interest rate, market and credit risks.  Due to the level of risk associated with certain investment securities, including Ecolab Inc. common stock, it is at least reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect participants’ account balances and the amounts reported in the 2012 statement of net assets available for benefits.

CONCENTRATION OF MARKET RISK:

At both December 31, 2012 and 2011, approximately 61% of the Plan’s net assets were invested in the common stock of Ecolab Inc. The underlying value of Ecolab Inc. common stock is dependent on the performance of Ecolab and the market’s evaluation of such performance.

DISTRIBUTIONS TO PARTICIPANTS:

Distributions to participants are recorded when paid.

PLAN EXPENSES:

The Company pays a portion of the administrative expenses of the Plan and a portion is paid by Plan participants within the Plan. Certain asset management and administrative fees of the Plan are charged against the Plan’s investment results.

NEW ACCOUNTING PRONOUNCEMENTS:

In May 2011, the Financial Accounting Standards Board (“FASB”) issued updated accounting guidance related to fair value measurements and disclosures that result in common fair value measurements and disclosures between accounting principles generally accepted in the United States of America (“U.S. GAAP”) and International Financial Reporting Standards.  This guidance includes amendments that clarify the application of existing fair value measurement requirements, in addition to other amendments that change principles or requirements for measuring fair value and for disclosing information about fair value measurements. The adoption of this guidance by the Plan in 2012 did not have a significant impact on the Plan’s financial statements or disclosures.

SUBSEQUENT EVENTS:

The Plan Administrator has evaluated subsequent events through the date and time the financial statements were issued.

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ECOLAB PUERTO RICO SAVINGS PLAN

NOTES TO FINANCIAL STATEMENTS

3.                          Investments:

Investments that represent 5 percent or more of the Plan’s net assets available for benefits at December 31, 2012 and 2011 are summarized as follows:

During 2012 and the period from December 12, 2011 to December 31, 2011, the Plan’s investments (including gains and losses on investments bought and sold, as well as held during the year) appreciated in value as follows:

4.                                Fair Value of Financial Instruments:

Accounting guidance establishes a framework for measuring fair value.  That framework provides a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (level 1 measurement) and the lowest priority to unobservable inputs (level 3 measurements). The three levels of the fair value hierarchy under accounting guidance are described below:

Level 1 - Unadjusted quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.

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ECOLAB PUERTO RICO SAVINGS PLAN

NOTES TO FINANCIAL STATEMENTS

4.                                Fair Value of Financial Instruments (continued):

Level 2 - Inputs other than quoted prices within Level 1 that are observable for the asset or liability, either directly or indirectly; and fair value is determined through the use of models or other valuation methodologies.  Investments which are generally included in this category include corporate bonds and loans, less liquid and restricted equity securities and certain over-the-counter derivatives.  If the asset or liability has a specific (contractual) term, the Level 2 input must be observable for substantially the full term of the asset or liability.  A significant adjustment to a Level 2 input could result in the Level 2 measurement becoming a Level 3 measurement.

Level 3 - Inputs are unobservable for the asset or liability and include situations where there is little, if any, market activity for the asset or liability.  The inputs into the determination of fair value are based upon the best information in the circumstances and may require significant management judgment or estimation.  Investments that are included in this category generally include equity and debt positions in private companies.

The asset’s or liability’s fair value measurement level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.  Valuation techniques used need to maximize the use of observable inputs and minimize the use of unobservable inputs.

The following is a description of the valuation methodologies used for investments measured at fair value.

Registered investment companies and Ecolab Inc. common stock: Investments in registered investment companies are recorded at the underlying net asset value per unit, which approximates fair value based on the publicly quoted market price of these funds. Investments in Ecolab Inc. common stock are recorded at fair value based on the quoted market price of Ecolab Inc.’s common stock on the New York Stock Exchange.

Common/collective trusts: Investments in common/collective trusts, with the exception of the investment in fully benefit-responsive investment contracts, are recorded at the underlying net asset value per unit, which approximates fair value based on the audited financial statements of these funds. The fair value of fully benefit-responsive investment contracts is calculated using a discounted cash flow model which considers recent fee bids as determined by recognized dealers, discount rate and the duration of  the underlying portfolio securities.

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ECOLAB PUERTO RICO SAVINGS PLAN

NOTES TO FINANCIAL STATEMENTS

4.                                Fair Value of Financial Instruments (continued):

The Plan reviews the fair value hierarchy classification on an annual basis. Changes in the ability to observe valuation inputs may result in a reclassification of levels for certain securities within the fair value hierarchy. The Plan’s policy is to recognize transfers into and out of levels within the fair value hierarchy at the end of the fiscal year in which the actual event or change in circumstances that caused the transfer occurs. There were no transfers between Level 1 and Level 2 during the years ended December 31, 2012 and 2011. When a determination is made to classify an asset or liability within Level 3, the determination is based upon the significance of the unobservable inputs to the overall fair value measurement. At December 31, 2012 and 2011, the Plan did not have any assets or liabilities classified within Level 3.

The methods described above may produce a fair value calculation that may not be indicative of net realizable value or reflective of future fair values. Furthermore, while the Plan believes its valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date.

The following tables represent the Plan’s fair value hierarchy for those assets measured at fair value on a recurring basis as of December 31, 2012 and 2011:

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ECOLAB PUERTO RICO SAVINGS PLAN

NOTES TO FINANCIAL STATEMENTS

4.                                Fair Value of Financial Instruments (continued):

5.                             Tax Status:

The Plan constitutes a qualified plan and trust under the Puerto Rico tax code. The Plan is maintained as a foreign trust covering only individuals subject to taxation under the Puerto Rico tax code and therefore no provision for United States federal income taxes has been included in the Plan’s financial statements.

A favorable determination letter, dated May 22, 2012, was received from the Puerto Rico Treasury Department on the form of the Banco Popular Master Defined Contribution Retirement Plan adoption agreement and master plan, upon which the Plan is maintained. The Company has also applied for an individual determination letter covering the Plan from the Puerto Rico tax authority. The Plan Administrator believes the Plan is currently designed and being operated in accordance with Section 1165(a) of the Puerto Rico Internal Revenue Code and is, therefore, exempt from Puerto Rico income taxes and, no provision for Puerto Rico income taxes has been included in the Plan’s financial statements.

U.S. GAAP requires plan management to evaluate tax positions taken by the Plan and recognize a tax liability (or asset) if the Plan has taken an uncertain position that more likely than not would not be sustained upon examination by the Puerto Rico tax authority. The Plan is subject to routine audits by taxing jurisdictions; however, there are currently no audits for any tax periods in progress.

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ECOLAB PUERTO RICO SAVINGS PLAN

NOTES TO FINANCIAL STATEMENTS

6.                             Related Party and Party-In-Interest Transactions:

The trustee is authorized under contract provisions, or by ERISA regulations providing an administrative or statutory exemption, to invest in funds under its control and in securities of Ecolab Inc.

Participant contributions and employer matching contributions are invested in one or more of the investment fund options offered under the Plan, including Ecolab Inc. common stock.

7.                          Reconciliation of Financial Statements to Form 5500:

The following is a reconciliation of net assets available for benefits per the financial statements at December 31, 2011 to the Form 5500:

7.                          Reconciliation of Financial Statements to Form 5500 (continued):

The following is a reconciliation of the net increase in net assets available for benefits per the financial statements for the year ended December 31, 2012 and for the period from December 12, 2011 to December 31, 2011 to the Form 5500:

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SUPPLEMENTAL SCHEDULES

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ECOLAB PUERTO RICO SAVINGS PLAN

SCHEDULE H, LINE 4i- SCHEDULE OF ASSETS (HELD AT END OF YEAR)

as of December 31, 2012

EIN 41-0231510

Plan Number: 005

(in thousands, except units)

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ECOLAB PUERTO RICO SAVINGS PLAN

SCHEDULE H, LINE 4i- SCHEDULE OF ASSETS (HELD AT END OF YEAR)

as of December 31, 2012 (Continued)

EIN 41-0231510

Plan Number: 005

(in thousands, except units)

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ECOLAB PUERTO RICO SAVINGS PLAN

SCHEDULE H, LINE 4i- SCHEDULE OF ASSETS (HELD AT END OF YEAR)

as of December 31, 2011

EIN 41-0231510

Plan Number: 005

(in thousands, except units)

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ECOLAB PUERTO RICO SAVINGS PLAN

SCHEDULE H, LINE 4i- SCHEDULE OF ASSETS (HELD AT END OF YEAR)

as of December 31, 2011 (Continued)

EIN 41-0231510

Plan Number: 005

(in thousands, except units)

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