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              [CREDIT SUISSE FIRST BOSTON CORPORATION LETTER HEAD]

                                                                    EXHIBIT 99.6

                                    CONSENT
                                       OF
                     CREDIT SUISSE FIRST BOSTON CORPORATION

Board of Directors
Valero Energy Corporation
One Valero Place
San Antonio, TX 78212-3186

Members of the Board:

Reference is made to our opinion letter dated May 4, 2001 (our "Opinion") with
respect to the merger of Valero Energy Corporation (the "Company") and Ultramar
Diamond Shamrock Corporation. Our Opinion is for the information of the Board of
Directors of the Company in connection with its consideration of such merger and
is not to be quoted or referred to, in whole or in part, in any registration
statement, prospectus or proxy statement, or in any other document used in
connection with the offering or sale of securities, nor for any other purpose,
without our prior written consent.

In that regard, we hereby consent to the inclusion of our Opinion as Appendix C
to the Registration Statement of the Company on Form S-4 filed with the
Securities and Exchange Commission on May 25, 2001 (the "Registration
Statement") relating to the transaction referred to above and references made to
such opinion in the Registration Statement under the captions (i)
Summary-Opinions of Financial Advisors; (ii) The Merger-Background of the
Merger; (iii) the Merger-Recommendation of the Valero Board of Directors;
Valero's Reasons for the Merger; and (iv) The Merger-Opinions of Valero's
Financial Advisors. In giving such consent, we do not admit that we come within
the category of persons whose consent is required under, nor do we admit that we
are "experts" for purposes of, the Securities Act of 1933, as amended, and the
rules and regulations promulgated thereunder.

CREDIT SUISSE FIRST BOSTON CORPORATION

/s/ CREDIT SUISSE FIRST BOSTON CORPORATION
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May 25, 2001