January 25, 1999




Metromedia Fiber Network, Inc.
1 North Lexington Avenue
White Plains, NY 10601


                         Registration Statement on Form S-4
                         of Metromedia Fiber Network, Inc.
                          ----------------------------------

Ladies and Gentlemen:

          In connection with the Registration Statement on Form S-4 (the
"Registration Statement") filed by Metromedia Fiber Network, Inc., a Delaware
corporation (the "Company"), with the Securities and Exchange Commission (the
"Commission") on the date of this letter, as provided by the Securities Act of
1933, as amended (the "Act"), and the rules and regulations under the Act, we
have been requested to render our opinion as to the legality of the securities
being registered under the Registration Statement.  The Registration Statement
relates to the registration under the Act of the Company's $650,000,000
aggregate principal amount of 



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10% Series B Senior Notes due 2008 (the "Exchange Notes").  The Exchange 
Notes are to be offered in exchange for the outstanding $650,000,000 
aggregate principal amount of 10% Series A Senior Notes due 2008 (the 
"Initial Notes") issued and sold by the Company on November 25, 1998 as part 
of an offering exempt from registration under the Act.  The Exchange Notes 
will be issued by the Company under the Indenture (the "Indenture"), dated as 
of November 25, 1998, between the Company and IBJ Whitehall Bank & Tust 
Company (formerly IBJ Schroder Bank & Trust 
Company), as trustee (the "Trustee"). Capitalized terms used in this letter 
and not otherwise defined have the respective meanings given those terms in 
the Registration Statement.

          In connection with this opinion, we have examined originals, conformed
copies or photocopies, certified or otherwise identified to our satisfaction, of
the following documents (collectively, the "Documents"):

          (i) the Registration Statement;

          (ii) the Indenture included as Exhibit 4.2 to the Registration
     Statement; 

          (iii)  the form of the Exchange Notes included as Exhibit 4.4 to the
     Registration Statement; and 

          (iv) The Registration Rights Agreement included as Exhibit 10.18 to
the Registration Statement. 

          In addition, we have examined: (i) those corporate records of the
Company as we have considered appropriate, including copies of its Amended and
Restated Certificate of Incorporation and Amended and Restated By-laws, as in
effect on the date of this letter (collectively, the "Charter Documents"), and
certified copies of resolutions of the executive committee of the board of
directors of the Company relating to the Exchange Notes; and (ii) those other
certificates, agreements and



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documents as we deemed relevant and necessary as a basis for the opinions
expressed below. 

          In our examination of the documents referred to above, we have
assumed, without independent investigation, (i) the due organization and valid
existence of the Company under the General Corporation Law of the State of
Delaware, (ii) the enforceability of the documents reviewed by us against each
party to them other than the Company, (iii) the necessary power and authority of
the Company to execute, deliver and perform its obligations under each of the
Documents to which it is a party, (iv) the due authorization, execution and
delivery by the Company of each of the Documents to which it is a party,
(v) that the authorization, execution and delivery  by the Company of each
Document to which it is a party and the consummation by the Company of the
transactions contemplated by them do not violate or result in a breach of or
default under the Company's Charter Documents, (vi) that the Exchange Notes will
be substantially issued as described in the Registration Statement and in the
form reviewed by us and that any information omitted from the form will be
properly added, (vii) the genuineness of all signatures, (viii) the authenticity
of all documents submitted to us as originals, (ix) the conformity to the
original documents of all documents submitted to us as certified, photostatic,
reproduced or conformed copies of validly existing agreements or other
documents, (x) the authenticity of the latter documents; (xi) that the
statements regarding matters of fact in the certificates, records, agreements,
instruments and documents that we examined are accurate and complete, and (xii)
the legal capacity of all individuals who have executed any of the documents
which we examined.



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     We have also assumed, without independent investigation, that (i) the
Indenture was duly authorized, executed and delivered by the Trustee, (ii) the
Indenture is a valid and binding obligation of the Trustee, (iii) the Exchange
Notes will be issued in accordance with the Indenture as described in the
Registration Statement and (iv) the Exchange Notes will be duly authenticated by
the Trustee in accordance with the Indenture.

          In expressing the opinion set forth below, we have relied upon the
factual matters contained in the representations and warranties of the Company
made in the documents and upon certificates of public officials and officers of
the Company. 

          Based on the foregoing, and subject to the stated assumptions,
exceptions and qualifications, we are of the opinion that:

          1.   The Indenture represents a valid and binding obligation of the
Company enforceable against the Company in accordance with its terms, except as
enforceability may be subject to (a) bankruptcy, insolvency, fraudulent
conveyance or transfer, reorganization, moratorium or other similar laws
affecting creditors' rights generally and (b) general principles of equity
(regardless of whether enforcement is considered in a proceeding in equity or at
law).

          2.   When issued, authenticated and delivered in accordance with the
terms of the Indenture, the Exchange Notes will be legal, valid and binding
obligations of the Company enforceable against the Company in accordance with
their terms, except as enforceability may be limited by (a) bankruptcy,
insolvency, fraudulent conveyance or transfer, reorganization, moratorium and
other similar laws affecting creditors' rights generally and (b) general
principles of equity (regardless of whether enforcement is considered in a
proceeding in equity or at law).



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          Our opinions expressed above are limited to the laws of the State of
New York, the General Corporation Law of the State of Delaware and the federal
laws of the United States of America.  Our opinions are rendered only with
respect to the laws, and the rules, regulations and orders under those laws,
that are currently in effect.  Please be advised that no member of this firm is
admitted to practice in the State of Delaware.

          We hereby consent to the use of our name in the Registration Statement
and in the prospectus in the Registration Statement as it appears in the caption
"Legal Matters" and to the use of this opinion as an exhibit to the Registration
Statement.  In giving this consent, we do not thereby admit that we come within
the category of persons whose consent is required by the Act or by the rules and
regulations under the Act.



                                   Very truly yours,

                              /s/ Paul, Weiss, Rifkind,
                                  Wharton & Garrison
 
                         PAUL, WEISS, RIFKIND, WHARTON & GARRISON