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                  [WILSON SONSINI GOODRICH & ROSATI LETTERHEAD]


                                                                    EXHIBIT 5.1


                                 August _, 2001


Komag, Incorporated
1710 Automation Parkway
San Jose, California 95131

        RE: REGISTRATION STATEMENT ON FORM S-8

Ladies and Gentlemen:

        We have examined the Registration Statement on Form S-8 to be filed by
you with the Securities and Exchange Commission on or about August 1, 2001(the
"Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, of 8,600,000 shares of your Common Stock
(the "Shares") to be issued under your 1988 Employee Stock Purchase, as amended,
1997 Supplemental Stock Option Plan, as amended, 1987 Restated Stock Option
Plan, as amended, and 2001 Employee Stock Purchase Plan (the "Plans"). As your
counsel in connection with this transaction, we have examined the proceedings
taken and are familiar with the proceedings proposed to be taken by you in
connection with the issuance of the Shares.

        It is our opinion that, when the shares are issued pursuant to options
and rights granted in the manner described in the Plans, and pursuant to the
agreements which accompany each option grant, the Shares will be legally and
validly issued, fully-paid and non-assessable.

        We consent to the use of this opinion as an exhibit to the Registration
Statement, and further consent to the use of our name whenever appearing in the
Registration Statement and any amendments thereto.


                                     Very truly yours,

                                     WILSON SONSINI GOODRICH & ROSATI
                                     Professional Corporation