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                                                                     EXHIBIT 5.1



                 [WILSON SONSINI GOODRICH & ROSATI LETTERHEAD]




                                  June 14, 2000



Komag, Incorporated
1710 Automation Parkway
San Jose, California 95131

        RE:    REGISTRATION STATEMENT ON FORM S-4

Ladies and Gentlemen:

        We have examined the Registration Statement on Form S-4 to be filed by
you with the Securities and Exchange Commission on or about June 14, 2000 (the
"Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, of shares of your Common Stock (the
"Shares"). 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 pursuant to the acquisition
transaction set forth and described in the Registration Statement.

        It is our opinion that, when issued in the manner described in the
Registration Statement, 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,

                                        /s/ Wilson Sonsini Goodrich & Rosati

                                        WILSON SONSINI GOODRICH & ROSATI
                                        Professional Corporation