The Washington Post Co. announced yesterday that it will join a Seattle-based partnership seeking federal permission to operate a new type of mobile telephone service in the Pacific Northwest.

The move is part of the Post Co.'s ongoing effort to enter that increasingly lucrative industry.

The partnership, Interstate Mobilephone Co., will apply for licenses from the Federal Communications Commission to operate cellular radio systems in Seattle, Portland, Ore., and about 10 other cities in the Northwest. The FCC will approve only two systems in a city -- one run by a telephone company and the other by a non-telephone company.

The partnership is one of several the Post Co. has entered into in applying for permission to offer the cellular service. Cellular radios use low-power transmitters and computers to enable several thousand consumers to talk simultaneously over mobile and portable telephones within a wide geographic area. Current mobile radio services, using high-power transmitters, allow only a handful of consumers to talk simultaneously.

In Washington and Baltimore area, the Post Co. has joined with American Radio-Telephone Service Inc. in its application to offer celluar service -- competing with four other applicants.

The Post Co. also is applying for a license in Miami as a partner with four local mobile-radio firms. On its own, it additionally is seeking a license in Detroit.

A company spokesman said the Post Co. still is looking at many other cities, including Jacksonville and Palm Beach, Fla., where it also may seek a license to offer cellular radio, on its own or in a partnership.