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                                                                     Exhibit 5.1




                                  April 3, 2000


i3 Mobile, Inc.
181 Harbor Drive, Third Floor
Stamford, Connecticut  06902

Gentlemen:

     As counsel for i3 Mobile, Inc., a Delaware corporation (the "Company"), we
are familiar with the Company's Registration Statement on Form S-1 (File No.
333-94191), first filed with the Securities and Exchange Commission (the "SEC")
under the Securities Act of 1933, as amended, (the "Act") on January 7, 2000, as
amended and supplemented (the "Registration Statement"), which relates to your
offering of 4,400,000 shares of the Company's Common Stock (together with
660,000 shares of Common Stock to cover over allotments, if any), par value
$0.01 per share (the "Shares").

     In connection with the foregoing, we have examined the Restated Certificate
of Incorporation of the Company filed as Exhibit 3.1 to the Registration
Statement and all amendments thereto, the Amended and Restated Bylaws of the
Company filed as Exhibit 3.7 to the Registration Statement and all amendments
thereto, the Underwriting Agreement in a form similar to the one filed as
Exhibit 1.1 to the Registration Statement, the Registration Statement and all
amendments thereto and such records, documents and other instruments as in our
judgment are necessary or appropriate to enable us to render this opinion.

     In our examination of the aforesaid documents, we have assumed, without
independent investigation, the genuineness of all signatures, the legal capacity
of all individuals who have executed any of the aforesaid documents, the
authenticity of all documents submitted to us as originals, the conformity with
originals of all documents submitted to us as copies (and the authenticity of
the originals of such copies) and the accuracy and completeness of all public
records reviewed by us. In making our examination of documents executed by
parties other than the Company, we have assumed that such parties had the power,
corporate or other, to enter into and perform all obligations thereunder, and we
have also assumed the due authorization by all requisite action, corporate or
other, and the valid execution and delivery by such parties of such documents
and the validity, binding effect and enforceability thereof with respect to such
parties.

     Based on the foregoing, we are of the opinion that the Shares have been
duly authorized for issuance and, after payment therefor in advance and in
accordance with the terms and provisions of the Underwriting Agreement and
issuance of the certificates therefor by the Company, will be duly and validly
issued, fully paid and nonassessable.

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     The foregoing opinion is rendered as of the date hereof. We assume no
obligation to update such opinion to reflect any facts or circumstances which
may hereafter come to our attention or changes in the law which may hereafter
occur. We hereby consent to the use of our name in the Registration Statement
and under the caption "Legal Matters" in the related Prospectus and consent to
the filing of this opinion as an exhibit to the Registration Statement.

                                              Very truly yours,



                                              PIPER MARBURY RUDNICK & WOLFE LLP