Exhibit 5.1

June 6, 1997


Ultramar Diamond Shamrock Corporation
9830 Colonnade Boulevard
San Antonio, Texas 78230

Re:  Registration Statement on Form S-3 of Ultramar Diamond Shamrock
     Corporation (Registration No. ______)

Gentlemen:

I am Executive Vice President, General Counsel, and Corporate Secretary of
Ultramar Diamond Shamrock Corporation (the "Company").  This letter is
delivered in connection with the registration, issuance, and sale of up to an
aggregate amount of $850,000,000 of Senior Debt Securities, Subordinated Debt
Securities, Common Stock, and Guarantees (collectively the "Offered
Securities") pursuant to the resolutions authorizing the issuance and sale of
the Offered Securities, and such other acts as are necessarily incident to
the registration, issuance, and sale of the Offered Securities (the
"Authorizing Resolutions") adopted by the Company's Board of Directors at a
meeting of the Company's Board of Directors held on June 4, 1997.  The Senior
Debt Securities will be issued pursuant to the terms of an indenture (the
"Indenture") dated as of March 15, 1995, between the Company and The Bank of
New York acting as trustee (the "Trustee").  The Subordinated Debt Securities
will be issued pursuant to the terms of an indenture (the "Subordinated Debt
Indenture") between the Trustee and the Company to be entered into.  

I have examined such documents, records, and matters of law as I have deemed
necessary for the purposes of this opinion, and based thereon I am of the
opinion that the Offered Securities have been duly authorized and will be
valid and binding obligations of the Company (except as enforcement thereof
may be limited by bankruptcy, insolvency, fraudulent conveyance,
reorganization, moratorium and other similar laws relating to or affecting
creditors' rights generally and subject to general equitable principles) when
the Offered Securities are issued, authenticated or countersigned, and
delivered by the Company for valid consideration in accordance with the
Authorizing Resolutions and (i) in the case of the Senior Debt Securities,
the terms of the Indenture, (ii) in the case of the Subordinated Debt
Securities, the terms of the Subordinated Debt Indenture, and (iii) in the
case of Common Stock, the Company's Certificate of Incorporation.

This opinion is based on the Company's Certificate of Incorporation and
Bylaws and applicable law as of the date hereof.  No assurance can be
provided as to the effect on this opinion of any amendment or other change to
the Company's Certificate of Incorporation or Bylaws or applicable law after
the date hereof.

I hereby consent to the filing of this opinion as an exhibit to the
Registration Statement on Form S-3 filed by the Company to effect
registration under the Securities Act of 1933, as amended, of the Offered
Securities and to the reference to me under the caption "Legal Matters" in
the Prospectus comprising part of such Registration Statement.

Very truly yours,


/s/ Patrick J. Guarino
    Patrick J. Guarino



PJG/hw
W5117.ASC