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                                                                     Exhibit 5.1

                       [WILSON SONSINI GOODRICH & ROSATI,
                      PROFESSIONAL CORPORATION LETTERHEAD]

                                October 10, 2000

Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303

     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 October 6, 2000 (the
"Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, of shares (the "Shares") of your Common
Stock, par value $0.00067 per share, to be issued in connection with your
acquisition of Cobalt Networks, Inc., a Delaware corporation, as described in
the Registration Statement (the "Transaction"). As your counsel in connection
with the 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 in the Transaction.

     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,

                                          WILSON SONSINI GOODRICH & ROSATI
                                          Professional Corporation

                                          /s/  Wilson Sonsini Goodrich & Rosati

Palo Alto, California