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                                                                     EXHIBIT 5.1


August 25, 2000


Metricom, Inc.
333 West Julian Street
San Jose, CA 95110


Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection
with the filing by Metricom, Inc. (the "Company") of a Registration Statement on
Form S-8 (the "Registration Statement") with the Securities and Exchange
Commission covering the offering of up to an aggregate of 3,300,000 shares of
the Company's Common Stock, $.001 par value per share, (the "Shares") pursuant
to its 1997 Equity Incentive Plan (the "Incentive Plan") and 1991 Employee Stock
Purchase Plan (the "Purchase Plan") (the Incentive Plan and Purchase Plan being
hereinafter collectively referred to as the "Plans").

In connection with this opinion, we have examined the Registration Statement and
related Prospectuses, your Certificate of Incorporation and By-laws, as amended
and restated, and such other documents, records, certificates, memoranda and
other instruments as we deem necessary as a basis for this opinion. We have
assumed the genuineness and authenticity of all documents submitted to us as
originals, the conformity to originals of all documents submitted to us as
copies thereof, and the due execution and delivery of all documents where due
execution and delivery are a prerequisite to the effectiveness thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion
that the Shares, when sold and issued in accordance with the Plans, the
Registration Statement and related Prospectuses, will be validly issued, fully
paid, and nonassessable (except as to shares issued pursuant to certain deferred
payment arrangements, which will be fully paid and nonassessable when such
deferred payments are made in full).

We consent to the filing of this opinion as an exhibit to the Registration
Statement.

Very truly yours,

Cooley Godward LLP


By: /s/ Kenneth L. Guernsey
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    Kenneth L. Guernsey