DRAFT:  March 25, 1997
                                                          ----------------------


               [Letterhead of Morris, Nichols, Arsht & Tunnell]

    
                                                                     Exhibit 5.1


                                March 26, 1997


USX Capital Trust I
c/o USX Corporation
600 Grant Street
Pittsburgh, Pennsylvania 15219

         Re:  USX Capital Trust I
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Ladies and Gentlemen:

         We have acted as special Delaware counsel to USX Capital Trust I,
a Delaware statutory business trust (the "Trust"), in connection with certain
matters relating to the creation of the Trust and the proposed issuance of
Convertible Preferred Securities of the Trust to beneficial owners pursuant to
and as described in Registration Statement Nos. 333-23291 and 333-23291-01 (and
the Prospectus forming a part thereof) on Form S-4 filed with the Securities
and Exchange Commission on March 14, 1997, as amended by Pre-Effective
Amendment No. 1 thereto (as so amended, the "Registration Statement").
Capitalized terms used herein and not otherwise herein defined are used as
defined in the Amended and Restated Declaration of Trust of the Trust in the
form attached as an exhibit to the Registration Statement (the "Governing
Instrument"). 

         In rendering this opinion, we have examined copies of the following
documents in the forms provided to us: the Certificate of Trust of the Trust as
filed in the Office of the Secretary of State of the State of Delaware (the
"State Office") on March 13, 1997 (the "Certificate"); a Declaration of Trust
of the Trust dated as of March 13, 1997 (the "Original Governing Instrument");
the Governing Instrument; the Multiple Series Indenture to be entered into
between the USX Corporation (the "Company") and The Bank of New York, as
Trustee (the "Base Indenture"); the Guarantee Agreement to be entered into by
the Company and The Bank of New York, as Trustee; the First Supplemental
Indenture, supplementing the Base Indenture, to be entered into between the
Company and The Bank of New York, as Trustee; the Dealer Manager Agreement
relating to the Convertible Preferred Securities among the Company, the Trust,
Goldman, Sachs & Co. and Merrill Lynch & Co. (the "Dealer Manager Agreement");
the Registration Statement; and a certificate of good standing of the Trust
obtained


 
USX Capital Trust I
March 26, 1997
Page 2

as of a recent date from the State Office. In such examinations, we have assumed
the genuineness of all signatures, the conformity to original documents of all
documents submitted to us as drafts or copies or forms of documents to be
executed and the legal capacity of natural persons to complete the execution of
documents. We have further assumed for purposes of this opinion: (i) the due
formation or organization, valid existence and good standing of each entity
(other than the Trust) that is a party to any of the documents reviewed by us
under the laws of the jurisdiction of its respective formation or organization;
(ii) the due authorization, execution and delivery by, or on behalf of, each of
the parties thereto of the above-referenced documents (including, without
limitation, the due authorization, execution and delivery of the Governing
Instrument and the Dealer Manager Agreement prior to the first issuance of
Convertible Preferred Securities); (iii) that no event has occurred subsequent
to the filing of the Certificate that would cause a dissolution or liquidation
of the Trust under the Original Governing Instrument or the Governing
Instrument, as applicable; (iv) that the activities of the Trust have been and
will be conducted in accordance with the Original Governing Instrument or the
Governing Instrument, as applicable; (v) that each Person that will acquire
Convertible Preferred Securities in the "Exchange Offer" (as defined in
Registration Statement and as used herein, the "Exchange Offer"), will validly
tender shares of 6.50% Convertible Preferred Stock of the Company in exchange
therefor, that such shares of 6.50% Convertible Preferred Stock will be duly
accepted, and that such Person will duly receive Convertible Preferred
Securities Certificates in consideration thereof, all in accordance with the
terms and conditions of the Governing Instrument, Registration Statement and the
Dealer Manager Agreement and that the Convertible Preferred Securities are
otherwise issued and sold to the Convertible Preferred Securities Holders in
accordance with the terms, conditions, requirements and procedures set forth in
the Governing Instrument, the Registration Statement and the Dealer Manager
Agreement; (vi) that the Company, as Sponsor, will issue and exchange Debentures
in the same aggregate principal amount as the aggregate stated liquidation
amount of the Common Securities and the Convertible Preferred Securities to be
exchanged therefor in accordance with the terms and conditions of the Governing
Instrument, the Registration Statement and the Dealer Manager Agreement; and
(vii) that the documents examined by us are in full force and effect, express
the entire understanding of the parties thereto with respect to the subject
matter thereof and have not been modified, supplemented or otherwise amended,
except as herein referenced. We have not reviewed any documents other than those
identified above in connection with this opinion, and we have assumed that
there are not other documents that are contrary to or inconsistent with the
opinions expressed herein. No opinion is expressed with respect to the
requirements of, or compliance with, federal or state securities or blue sky
laws. We express no

USX Capital Trust I
March 26, 1997
Page 3
 
 
 
opinion as to, and assume no responsibility for, the Registration Statement or
any other offering materials relating to the Convertible Preferred Securities.
As to any fact material to our opinion, other than those assumed, we have
relied without independent investigation on the above-referenced documents and
on the accuracy, as of the date hereof, of the matters therein contained.

     Based on and subject to the foregoing, and limited in all respects to
matters of Delaware law, it is our opinion that:

     1.  The Trust is a duly created and validly existing business trust in good
standing as a business trust under the laws of the State of Delaware.
 
     2.  The Convertible Preferred Securities, upon issuance pursuant to the 
Exchange Offer, will constitute validly issued and, subject to the 
qualifications set forth in paragraph 3 below, fully paid and nonassessable 
beneficial interests in the assets of the Trust.

     3.  Under the Delaware Business Trust Act, 12 Del. C. section 3801 et. seq.
                                                   -------              --------
and the terms of the Governing Instrument, each Convertible Preferred Security 
Holder of the Trust, in such capacity, will be entitled to the same limitation 
of personal liability extended to stockholders of private corporations for 
profit organized under the General Corporation Law of the State of Delaware; 
provided, however, that we express no opinion with respect to the liability of 
any Convertible Preferred Security Holder who is, was or may become a named 
Trustee of the Trust. Notwithstanding the foregoing, we note that (a) pursuant
to Section 11.4 of the Governing Instrument, the Trust may withhold amounts
otherwise distributable to a Holder and pay over such amounts to the applicable
jurisdictions in accordance with federal, state and local law and any amount
withheld will be deemed to have been distributed to such Holder and (b) pursuant
to the Governing Instrument, Convertible Preferred Security Holders may be
obligated to make payments or provide indemnity or security under the
circumstances set forth therein.

     We hereby consent to the filing of this opinion as an exhibit to the 
Registration Statement and to the use of our name and reference to our opinion 
under the heading "LEGAL MATTERS" in the Prospectus forming a part thereof. In 
giving this consent, we do not thereby admit that we come within the category of
persons whose consent is required under Section 7 of the Securities Act of 1933,
as amended, or the rules and regulations of the Securities and Exchange 
Commission thereunder. This opinion speaks only as of the date hereof and is 
based on our understandings and assumptions as to present facts, and on our 
review of the above-referenced documents and the application of Delaware law as 
the same exist as

USX Capital Trust I
March 26, 1997
Page 4

of the date hereof, and we undertake no obligation to update or supplement this
opinion after the date hereof for the benefit of any person or entity with
respect to any facts or circumstances that may hereafter come to our attention
or any changes in facts or law that may hereafter occur or take effect. This
opinion is intended solely for the benefit of the addressee hereof in connection
with the matters contemplated hereby and may not be relied on by any other
person or entity or for any other purpose without our prior written consent.

                                           Very truly yours,


                                           MORRIS, NICHOLS, ARSHT & TUNNELL


                                           /s/ Morris, Nichols, Arsht & Tunnell