EXHIBIT 99.1

FOR IMMEDIATE RELEASE

                  CONTACTS:
                  Valero Energy Corporation          UDS
                  Mary Rose Brown (Media)            Tara Ford (Media)
                  (210) 370-2314                     (210) 592-4163
                  Lee Bailey (Investor Relations)    Scott Spendlove (Inv. Rel.)
                  (210) 370-2139                     (210) 592-4019

         VALERO ENERGY CORP. TO ACQUIRE ULTRAMAR DIAMOND SHAMROCK CORP.

      $6 Billion Transaction To Create Second-Largest U.S. Refining Company

SAN ANTONIO,  May 7, 2001 - Valero Energy Corp. (NYSE: VLO) and Ultramar Diamond
Shamrock Corp.  (NYSE:  UDS) jointly announce that the companies have reached an
agreement for Valero to acquire UDS,  which will make Valero the  second-largest
United States  refiner of petroleum  products.  The new  organization  will have
23,000  employees in the United  States and Canada,  13  refineries  and a total
throughput  capacity of just under 2 million barrels per day (BPD).  Valero will
also be one of the  nation's  largest  retailers  with  more than  5,000  retail
outlets in the U.S. and Canada.

With this acquisition, Valero will have annual revenues of $32 billion and total
assets of more than $10 billion. In terms of refining capacity,  Valero will now
be second only to ExxonMobil.

The  transaction is expected to be highly  accretive to earnings,  cash flow and
returns based on current First Call consensus estimates.

"We're  combining the two best independent  refining and marketing  companies to
make the premier refiner and marketer in the U.S.," said Bill Greehey,  Valero's
chairman of the board and CEO. "Obviously this will bring tremendous benefits to
both of our  organizations  and to our  shareholders.  In fact,  in refining and
marketing,  we will be the only major  independent  of a size and scope equal to
the majors.



"Combining  Valero's  complex  refining  system and the  extensive UDS refining,
logistics and retail network gives us a superior asset portfolio that will allow
us to  effectively  compete  in this  rapidly  consolidating  business.  We will
realize  tremendous  synergies and strategic  benefits while enhancing  earnings
stability.  And, we are fortunate to bring aboard such a high-quality management
team and workforce that is among the best in the industry."

The total value of the transaction is approximately  $6 billion,  which includes
approximately  $4 billion in equity  and $2 billion in assumed  debt.  The total
consideration  to be paid to UDS  stockholders in the  transaction  equates to a
fixed  exchange  ratio of 1.228 shares of Valero common stock for  approximately
half of the outstanding  shares of UDS common stock, and $55 dollars in cash per
share for the remaining shares of UDS common stock. The cash  consideration  and
the stock  consideration per UDS share each represent an approximate 30% premium
to UDS  shareholders  based on the 10-day  average per share prices ending April
26th.  UDS  shareholders  will be able to elect to receive either cash or Valero
common stock for each share of UDS common stock they own, subject to pro-ration,
with the consideration,  whether paid in cash or stock,  having a value equal to
the average of $55.00 and the value of 1.228 shares of Valero  common stock over
a measurement period prior to closing.

"This really is a winning  combination for our stockholders,"  said Jean Gaulin,
chairman  of the  board  and  CEO of  UDS.  "UDS  shareholders  will  receive  a
substantial premium,  which reflects a more appropriate  valuation for the stock
and captures  value that the market has otherwise  been slow to recognize.  They
will also lock in a solid return with the cash portion. Valero shareholders will
benefit from a transaction  that is immediately and  significantly  accretive to
earnings and cash flow per share.  Since UDS shareholders will own 40 percent of
the  company,  both  sets  of  shareholders  will  benefit  from  the  increased
profitability  of the company going forward.  And because larger  companies have
more liquidity and  visibility,  we are optimistic that the market will assign a
higher multiple valuation to the new company."



Refining and Logistics Assets

With  the  addition  of the UDS  refining  assets,  Valero  will  have  the most
geographic diversity among U.S. refiners. The seven UDS refineries, which have a
combined  capacity of 850,000 BPD,  will expand  Valero's  presence in Texas and
California,  while giving it new  locations  in  Colorado,  Oklahoma and Quebec,
Canada.  Like Valero,  the UDS  refineries  are also  generally  high-conversion
facilities which produce premium,  cleaner-burning fuels including  reformulated
gasoline,  California's  cleaner-burning  CARB gasoline and CARB diesel,  all of
which sell at a premium  to  conventional  gasoline.  Valero  will also  acquire
Ultramar Diamond Shamrock's extensive,  4,600-mile proprietary pipeline network,
including the Shamrock  Logistics  MLP and  associated  assets,  that reduce the
refineries' feedstock supply costs and distribute products at a higher netback.

"We will benefit from enhanced  financial  performance  through  realization  of
numerous  potential  synergies  among the facilities,  including  multi-refinery
purchasing and inventory  optimization,"  said Greehey.  "We estimate that these
synergies will have a benefit in excess of $200 million annually."

Greater Exposure to Positive Fundamentals

The transaction also comes amid the very strong refining  industry  fundamentals
that have helped both companies achieve record earnings recently.

"The current industry  fundamentals  and the long-term  outlook for our business
have never been better," said Greehey.  "Refined product inventories continue to
trend at  historically  low levels.  At the same time,  gasoline and  distillate
demand remain strong.  With limited excess refining  capacity in the U.S., there
is very little room for inventories to build  substantially  in the near future.
This should continue to keep the supply/demand balance tight and support healthy
refining margins going forward.



"We continue to see gasoline demand better than last year's levels and expect to
see a good driving season," he added.  "And now more than ever, Valero is poised
to seize the greatest benefits from these fundamentals  because we will continue
to have the most leverage to improved refining margins."

Marketing Assets

Branded Ultramar,  Diamond  Shamrock,  Beacon and Total, UDS has more than 2,500
company-owned  sites  throughout the West,  Southwest and  Mid-continent  of the
United States and in Eastern Canada, which are among the fastest growing markets
in the country. The company also supplies approximately 2,500 dealer, truck stop
and cardlock sites primarily in the Southwest and  Mid-continent  regions of the
United States and in Eastern Canada.

The  company-owned   stores  have  extensive  brand  support  programs  such  as
proprietary consumer and fleet credit cards, radio and television brand support,
and strong in-store marketing programs,  to which Valero will be able to add its
350-store retail network in California.

In addition,  UDS operates  one of the largest  home heating oil  businesses  in
North America that sells heating oil to approximately 250,000 households.

"We're excited to substantially  grow our retail presence because retail margins
are  counter-cyclical  to refining margins,  so in the event we experience lower
refining  margins,  retail  margins  will help  stabilize  our  earnings,"  said
Greehey.  "We are  fortunate  because  UDS has  created  one of the best  retail
networks in the country with excellent leadership and strategic vision. And with
the size and quality of their retail network,  we will benefit from the earnings
stability that can be achieved by having retail  exposure  without  diluting the
leverage  we have to  refining  margins.  So we will truly have the best of both
worlds."



Timing, Transition and Future

Greehey  will  remain  chairman  and CEO of Valero  and the  company's  board of
directors  will grow to 13  members  with the  addition  of four UDS  directors.
Gaulin will  continue to run UDS until the closing and will work with Greehey on
the  organization of the new Valero.  He will then retire and step down from the
board of directors.

The boards of directors of both companies have approved the  transaction,  which
is  subject to the  approval  of both  companies'  shareholders  and  regulatory
approvals.  The  acquisition  is  expected  to  close by the end of the year and
Greehey noted that a smooth  transition  is  anticipated.  Morgan  Stanley & Co.
Incorporated  is serving as Valero's  financial  advisor and provided a Fairness
Opinion  letter  related to the  transaction,  and Credit Suisse First Boston is
also providing a Fairness Opinion letter to Valero.  Banc of America Securities,
LLC is advising UDS.

"Valero has  demonstrated  that we deliver on the  transactions  we announce and
that we integrate those  acquisitions  smoothly and effectively,"  Greehey said.
"The fact that UDS is in San Antonio will  contribute to making this  transition
even  easier.  The  combined  company  will benefit from having the best of both
management teams and the assets to build upon for the future.

"And, we're not done! With the financial capacity we'll have, we fully intend to
expand our  business,  grow the  company and  continue  to deliver  value to our
shareholders."

Valero Energy Corporation

Valero is a Fortune 500 company based in San Antonio,  with approximately  3,100
employees and 2000 revenues of nearly $15 billion.  The company  currently  owns
and operates six refineries in Texas, Louisiana,  New Jersey and California with
a combined  throughput capacity of more than 1 million BPD. Valero is recognized
throughout   the   industry   as  a  leader  in  the   production   of  premium,
environmentally  clean  products such as  reformulated  gasoline,  CARB Phase II
gasoline,  low-sulfur diesel and oxygenates. The company markets its products in
34 states through an extensive bulk and rack  marketing  network,  in California
through  approximately 350 retail  locations,  and in selected export markets in
Latin America.



Ultramar Diamond Shamrock Corporation

Ultramar Diamond Shamrock Corp.  (NYSE:UDS),  has  approximately  $17 billion in
annual  revenues  and more than 20,000  employees.  The company  operates  seven
refineries in the United States and Canada with a total  throughput  capacity of
850,000 barrels per day and has nearly 5,000 branded retail gasoline/convenience
merchandise  stores,  the  majority  of  which  are  branded  Diamond  Shamrock,
Ultramar,  Beacon or Total. The corporation also has growing  petrochemicals and
home heating oil businesses.
                                      -30-
WEBCAST - An analyst meeting is scheduled for 10 a.m. EDT Monday,  May 7, at the
St.  Regis  Hotel  in  New  York  City.  To  listen,  go  to  www.valero.com  or
www.udscorp.com,  enter  the  Investor  Relations  page and  click on the  "Live
Webcast"  link. A replay of this  broadcast  will be available on both  websites
following the presentation.

SATELLITE FEED - A satellite feed will be available on Monday, May 7 at 12-12:15
p.m.  (EDT) and 5-5:15 p.m.  (EDT) on Galaxy 4R/C20 and K16. The video  includes
sound  bites  from the CEO's of both  companies,  as well as b-roll of  Valero's
refineries  and  Ultramar  Diamond   Shamrock's  retail  stores.  For  satellite
assistance, call (210) 377-8666.

Statements  contained  in  this  press  release  that  state  the  Company's  or
management's  expectations  or  predictions  of the future  are  forward-looking
statements  intended  to be  covered  by  the  safe  harbor  provisions  of  the
Securities Act of 1933 and the Securities  Exchange Act of 1934. It is important
to note that the Company's  actual  results could differ  materially  from those
projected in such forward-looking statements.

Investors   and   security   holders   are  urged  to  read  the   joint   proxy
statement/prospectus  that will be sent to Valero and UDS stockholders regarding
the  proposed  merger,  when it  becomes  available,  because  it  will  contain
important information.  The joint proxy  statement/prospectus will be filed with
the Securities and Exchange Commission by Valero and UDS. Investors and security
holders may obtain a free copy of the joint proxy statement/prospectus,  when it
is available, and other documents filed by Valero and UDS with the Commission at
the Commission's web site at www.sec.gov.  The joint proxy  statement/prospectus
and these other documents may also be obtained,  when available,  free of charge
from  Valero  and UDS.  Stockholders  should  read the  definitive  joint  proxy
statement/prospectus carefully before making a decision concerning the merger.

Valero and UDS, and their respective  directors,  executive officers and certain
other of their  respective  employees,  may be  soliciting  proxies  from  their
respective  stockholders  in favor of the  approval of the  merger.  Information
regarding the persons who may, under SEC rules,  be deemed to be participants in
the solicitation of Valero and UDS stockholders in connection with the merger is
set forth,  in the case of Valero,  in  Valero's  proxy  statement  for its 2001
annual meeting, filed with the SEC on March 28, 2001, and in the case of UDS, in
UDS's proxy statement for its 2001 annual  meeting,  filed with the SEC on March
27, 2001, and additional  information  will be set forth in the definitive proxy
statement/prospectus referred to above when it is filed with the SEC.