A Fairfax County Circuit Court judge refused yesterday to close a Mount Vernon adult bookstore, but ordered the business to rip out partitions around 60 booths that witnesses have said are the scenes of illegal sexual encounters.

Judge Barbara M. Keenan rejected prosecutors' attempts to order the closing of Show Place Adult Books on Richmond Highway under a Virginia nuisance law previously used to shut down houses of prostitution and massage parlors.

She said that even though the sexual activities that have occurred in the store create a public nuisance, closing the business would infringe on First Amendment rights covering the books and movies sold and rented there.

The judge cautioned the store's attorneys that she expects the sexual activities to cease. "The court is not suggesting the First Amendment can be used as a defense to hide behind continually."

Both prosecutors and the bookstore's attorneys said later they plan to appeal the judge's decision to the state Supreme Court. Prosecutors said they will base their appeal on grounds that the nuisance statute is a constitutional means of closing the store and the store's attorneys said they will contend that the judge's orders to modify the store infringe on the owners' rights to conduct their business.

Prosecutors and Fairfax officials have sought for years to close the store, which is operated by a Virginia corporation, Croatan Books Inc. The corporation has been convicted of selling or renting obscene materials at least 79 times since 1975, according to court records. A separate effort to force the store to cease operations is pending before the State Corporation Commission, where the state attorney general's office is seeking to dissolve Croatan's charter on the grounds that it has exceeded or abused the authority granted it.

Yesterday's hearing followed one last month in which several of the store's customers testified that they had been solicited for homosexual favors there and had engaged in sex acts with others in movie projection booths at the rear of the store.

Keenan said she thinks the modifications will curtail the sexual activity and she ordered store owners to appear in court again Feb. 17 to determine if her order is effective.

"If it does not eliminate the complaints," said Keenan, "the court retains the right to consider more drastic alternatives, including closing the premises."

Assistant Commonwealth's Attorney Richard F. MacDowell said, "I'm pretty skeptical that it's going to solve the problem. The store will continue to be a meeting place for those kinds of people."

Croatan lawyer Burton W. Sandler said he will appeal, contending that the order infringes on the store's First Amendment rights and will severely curtail its business because the store will be unable to provide the movies for several months during renovations..

Sandler said he believes the order is too drastic and that Keenan's temporary order issued in June posting guards in the store was adequate to control the problems in the store.

"It seems be be effective because we've lost a lot of business," Sandler said.