EXHIBIT 5.1

October 3, 2000


Ultramar Diamond Shamrock Corporation
P. O. Box 696000
San Antonio, Texas  78269-60000

Re:  Ultramar  Diamond  Shamrock  Corporation Non-Employee  Director Equity Plan
     (the "Plan")

Gentlemen:

I am Counsel for Ultramar Diamond Shamrock  Corporation,  a Delaware corporation
(the  "Company").  The Company  expects to file with the Securities and Exchange
Commission  on or about  October 5, 2000 under the  Securities  Act of 1933,  as
amended,  a Registration  Statement on Form S-8/A (the Registration  Statement")
for the purpose of registering  250,000 shares of common stock,  $0.01 par value
of the Company ("Common stock").

In  connection  with  such  filing,  I have  examined  the Plan  and such  other
documents,  records,  and matters of law as I have deemed necessary for purposes
of this  opinion and based  thereupon,  I am of the  opinion  that the shares of
Common stock that may be issued and sold or delivered  pursuant to the Plan will
be, when issued in accordance  with the provisions of the Plan,  legally issued,
fully  paid,  and  nonassessable  and the rights to purchase  common  stock (the
"Rights") in accordance  with the Rights  Agreement  dated June 25, 1992 between
the Company and  Registrar  and  Transfer  Company,  when duly  issued,  will be
legally issued.

I hereby consent to the filing of this opinion as an exhibit to the Registration
Statement  on Form S-8/A for the Plan filed by the Company  with the  Securities
and Exchange  Commission to effect  registration  of such Common Stock under the
Securities Act of 1933, as amended.

Very truly yours,

/s/ Todd Walker
    Todd Walker