Soviet inspectors who'll be in Utah under provisions of the Intermediate Nuclear Forces Treaty will dress in civilian clothes and keep a low profile, a spokesman for the new On-Site Inspection Agency said Saturday in Washington.

And U.S. and Soviet reporters will not be allowed to watch each other's inspectors at work.A team of 40 Soviet inspectors is to live in Utah near Magna to observe the Hercules Inc. plant to be sure it does not make more Pershing missiles, as directed by the new treaty.

The United States has been readying a team of American inspectors to make a dry run at Soviet and East European missile sites in preparation for actual on-site checks once the treaty is ratified by Congress and the Supreme Soviet.

The treaty eliminates intermediate-range nuclear missiles deployed by the Soviets and the United States in Europe, and prohibits building more of them.

President Reagan and Soviet leader Mikhail Gorbachev signed the treaty during a Washington summit in December. Ratification is expected to be certified by the two nations during a May summit meeting in Moscow between the two leaders.

Soviet inspectors have already visited Salt Lake City to look over living and other arrangements at Magna.