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                                                                     EXHIBIT 8.1

                    [Letterhead of Willkie Farr & Gallagher]

August 29, 2001



XO Communications, Inc.
11111 Sunset Hills Road
Reston, VA 20190

Ladies and Gentlemen:

We are delivering this opinion in connection with the Registration Statement on
Form S-3 (the "Registration Statement") initially filed by XO Communications
Inc. (the "Company"), on August 17, 2001, with the Securities and Exchange
Commission under the Securities Act of 1933, as amended (the "Act"), with
respect to (i) $517,500,000 aggregate principal amount at maturity of
convertible subordinate notes due 2009 issued by the Company on January 12, 2001
(collectively, the "Notes"), and (ii) shares of the Company's class A common
stock, $0.02 par value per share (the "Shares"), issuable upon conversion of the
Notes. The Notes have been issued pursuant to an Indenture, dated as of January
12, 2001, between the Company and U.S. Trust Company, National Association, as
trustee (the "Indenture"). The Notes and the Shares are to be offered and sold
by certain selling securityholders of the Company from time to time as set forth
in the Registration Statement, and any amendments or supplements thereto.

We have examined and are familiar with originals or copies, certified or
otherwise identified to our satisfaction, of such documents, corporate records
and other instruments relating to the incorporation of the Company and to the
authorization and issuance of the Notes and the Shares, including the Indenture
and Registration Statement, and have made such investigations of law, as we have
deemed necessary and advisable. In such examination, we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to us
as originals and the conformity to authentic originals of all documents
submitted to us as copies.

This opinion is based on relevant provisions of the Internal Revenue Code of
1986, as amended, the Treasury Regulations issued thereunder, court decisions,
and administrative determinations as currently in effect, all of which are
subject to change, prospectively or retroactively, at any time. This opinion is
rendered as of the date hereof and we undertake no obligation to update or
supplement this opinion to reflect any changes in laws that may occur after the
date hereof.

Based upon the foregoing and subject to the qualifications and limitations
stated herein, we are of the opinion that the statements under the caption
"Certain United States Federal Income Tax Consequences" in the prospectus
included in the Registration Statement, insofar as such


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August 29, 2001
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statements constitute a summary of the United States federal tax laws referred
to therein, are accurate and fairly summarize in all material respects the
United States federal tax laws referred to therein.

We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement referred to above and to the reference to our firm under
the caption "Certain United States Federal Income Tax Consequences" in the
prospectus included in the Registration Statement. We do not admit by giving
this consent that we are in the category of persons whose consent is required
under Section 7 of the Act.

The foregoing opinion is limited to the U.S. federal tax matters addressed
herein, and no other opinions are rendered with respect to other federal tax or
other matters or to any issues arising under the tax laws of any other country,
or any state or locality.


Very truly yours,

/s/ Willkie Farr & Gallagher