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                                                                     EXHIBIT 5.1



                  [COTTON, BLEDSOE, TIGHE & DAWSON LETTERHEAD]


                                 April 15, 1999


Costilla Energy, Inc.
400 West Illinois, Suite 1000
Midland, Texas 79701

         Re:      Registration Statement on Form S-3

Gentlemen:

         We have acted as counsel for Costilla Energy, Inc., a Delaware
corporation (the "Company") in connection with the registration under the
Securities Act of 1933, as amended (the "Act"), of 1,000,000 shares of the
Company's Common Stock, $0.10 par value (the "Shares"). A Registration Statement
on Form S-3 covering the sale of the Shares was filed under the Act with the
Securities and Exchange Commission (the "Commission") on or about April 15, 1999
(the "Registration Statement").

         In reaching the conclusions expressed in this opinion, we have examined
signed copies of the Registration Statement and all exhibits thereto. We have
also examined and relied upon originals, or copies certified to our
satisfaction, of (i) the Certificate of Incorporation and Bylaws of the Company,
(ii) minutes and records of the corporate proceedings of the Company with
respect to the issuance of the Shares and related matters, and (iii) such other
agreements and instruments relating to the Company as we have deemed necessary
or appropriate for the purposes of the opinions hereinafter expressed. In
rendering such opinions, we have relied, to the extent we deemed reasonable, on
certificates and certain other information provided to us by officers of the
Company and public officials as to matters of fact of which the maker of such
certificates or the person providing such information had knowledge, without
investigation into or verification of such information. Furthermore, we have
assumed that the signatures on all documents examined by us are genuine, that
all documents and corporate record books submitted to us as originals are
authentic, accurate and complete, and that all documents submitted to use as
copies are true, correct and complete copies of the originals thereof.



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Costilla Energy, Inc.
April 15, 1999
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         Based solely upon the foregoing, subject to the assumptions,
limitations and qualifications set forth herein, and specifically limited in all
respects to the laws of the State of Texas, of the United States of America and
the General Corporation Law of the State of Delaware, we are of the opinion that
the Shares registered pursuant to the Registration Statement have been duly and
validly authorized by the Company and have been legally issued, fully paid and
nonassessable. Please note in this regard that we are not licensed to practice
law in the State of Delaware, but have reviewed Delaware law in connection with
the opinions expressed herein.

         We hereby consent to the use of this opinion as an exhibit to the
Registration Statement and to the reference to this Firm under the caption
"Legal Matters" in the Prospectus forming a part of the Registration Statement.
In giving this consent we do not thereby admit that we come within the category
of persons whose consent is required under the Act or the rules and regulations
of the Commission promulgated thereunder.

         This opinion is rendered only to the Company and solely for the benefit
of the Company and the Commission in connection with the registration of the
Shares pursuant to the Registration Statement. This opinion may not be otherwise
used, circulated, quoted, relied upon, or referred to by you or the Commission
for any other purpose or by any other person, firm or corporation for any
purpose, without our prior written consent.



                                               Yours very truly,

                                               COTTON, BLEDSOE, TIGHE & DAWSON


                                               By: /s/ Dan G. LeRoy
                                                  ----------------------------
                                                       Dan G. LeRoy