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                                                                    EXHIBIT 99.a
       ONEOK FINANCIAL NEWS
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ONEOK, INC.                                 CONTACT: WELDON WATSON, 918/588-7158
P. O. BOX 871
TULSA, OK 74102-0871                        FOR IMMEDIATE RELEASE APRIL 20, 1999



                  ONEOK/COSTILLA STRATEGIC ALLIANCE TERMINATES


         Tulsa, Oklahoma - The previously announced strategic alliance between
ONEOK, Inc., NYSE:OKE) and Costilla Energy, Inc., (Nasdaq:COSE) has been
terminated after the announcement by Costilla that it had terminated a purchase
sale agreement with Pioneer Natural Resources Company (NYSE:PXD).

         The strategic alliance was contingent upon the success of Costilla's
agreement to acquire $335 million in certain oil and gas properties owned by
Pioneer Natural Resources. The ONEOK alliance with Costilla involved a $35
million acquisition of properties and a $65 million equity investment in
Costilla.

         ONEOK, Inc., is an integrated natural gas company involved in
production, processing, gathering, storage and transmission. The company is also
the largest natural gas distributor in Kansas and Oklahoma operating as Oklahoma
Natural Gas Company and Kansas Gas Service Company, serving 1.4 million
customers.

                                      # # #

STATEMENTS CONTAINED IN THIS RELEASE THAT INCLUDE COMPANY 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 ACTUAL RESULTS OF COMPANY
EARNINGS COULD DIFFER MATERIALLY FROM THOSE PROJECTED IN SUCH FORWARD-LOOKING
STATEMENTS. INFORMATION IS AVAILABLE ON THE INTERNET WORLD WIDE WEB AT
http://www.oneok.com.





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