<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>
<periodOfReport>2003-11-01</periodOfReport>
<notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0000031462</issuerCik>
        <issuerName>ECOLAB INC</issuerName>
        <issuerTradingSymbol>ECL</issuerTradingSymbol>
</issuer>
<reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001189430</rptOwnerCik>
            <rptOwnerName>LEVIN JERRY W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>AMERICAN HOUSEHOLD, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>2381 EXECUTIVE CENTER DRIVE</rptOwnerStreet2>
            <rptOwnerCity>BOCA RATON</rptOwnerCity>
            <rptOwnerState>FL</rptOwnerState>
            <rptOwnerZipCode>33431</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector>
<isOfficer>0</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle></officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeTable><nonDerivativeHolding><securityTitle><value>Common Stock</value>
</securityTitle>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>45851.27</value>
<footnoteId id="F1"></footnoteId>
<footnoteId id="F2"></footnoteId>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>D</value>
</directOrIndirectOwnership>
</ownershipNature>
</nonDerivativeHolding>
</nonDerivativeTable>
<derivativeTable><derivativeTransaction><securityTitle><value>Stock Option (Right to Buy)</value>
</securityTitle>
<conversionOrExercisePrice><value>27.07</value>
</conversionOrExercisePrice>
<transactionDate><value>2003-11-01</value>
</transactionDate>
<transactionCoding><transactionFormType>4</transactionFormType>
<transactionCode>A</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionAmounts><transactionShares><value>5717</value>
</transactionShares>
<transactionPricePerShare><footnoteId id="F3"></footnoteId>
</transactionPricePerShare>
<transactionAcquiredDisposedCode><value>A</value>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<exerciseDate><value>2003-11-01</value>
</exerciseDate>
<expirationDate><value>2013-11-01</value>
</expirationDate>
<underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value>
</underlyingSecurityTitle>
<underlyingSecurityShares><value>5717</value>
</underlyingSecurityShares>
</underlyingSecurity>
<postTransactionAmounts><sharesOwnedFollowingTransaction><value>5717</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>D</value>
</directOrIndirectOwnership>
</ownershipNature>
</derivativeTransaction>
</derivativeTable>
<footnotes><footnote id="F1">Includes 103.11 shares acquired pursuant to a dividend reinvestment feature of the Ecolab Inc. 2001 Non-Employee Director Stock Option and Deferred Compensation Plan.</footnote>
<footnote id="F2">The number of shares owned by the reporting person has been doubled to reflect the 2 for 1 stock split of Ecolab Inc. Common Stock effected in the form of a 100% stock dividend on June 6, 2003.</footnote>
<footnote id="F3">Option granted in lieu of $38,684.43 of Non-employee Director compensation earned by reporting person over the period from May 10, 2003 to November 1, 2003.</footnote>
</footnotes>
<ownerSignature><signatureName>/s/David F. Duvick, Power of Attorney for Jerry Levin</signatureName>
<signatureDate>2003-11-04</signatureDate>
</ownerSignature>
</ownershipDocument>

