A Fairfax County Circuit Court judge ruled yesterday that a Mount Vernon area adult bookstore is a public nuisance because individuals gather there for illicit sexual activities, but stopped short of ordering the store closed.

Judge Barbara M. Keenan postponed a decision until Aug. 2 on how to end the nuisance at Show Place Adult Books on Richmond Highway, a store that prosecutors and county officials have sought for years to close.

Lawyers in the case say they believe her ruling represents the first time Virginia's 64-year-old nuisance law, used previously to close houses of prostitution and massage parlors, has been invoked in an attempt to close an adult bookstore.

Keenan issued the ruling after several of the store's patrons testified that they had been solicited for sexual favors while in the store and had engaged in sodomy with others in movie projection booths at the rear of the store.

"You are almost constantly solicited for sex, virtually every time you go to the store," said the 40-year-old president of a Washington business firm, who testified he had engaged in homosexual activities about 15 to 20 times in projection booths there.

"It is understood the movies are there for sexual solicitation."

Attorneys for the store, operated by a Virginia corporation known as Croatan Books Inc., argued that shutting down the entire business because of alleged activities between consenting adults in one part of the store would infringe on the firm's First Amendment rights.

"This is not a house of prostitution," said Croatan lawyer Burton W. Sandler. "This is a bookstore. The effect of this action sought by the commonwealth can be a gross denial of due process of law."

The judge asked prosecutors and defense attorneys to attempt to reach a compromise to curtail the sexual activities at the store before the Aug. 2 hearing. They failed to reach an agreement during a 90-minute closed-door meeting in Keenan's chambers yesterday.

Defense attorneys said they may be willing to agree to a compromise modifying some aspects of the store's operations, but said they still believe the proposal to close the store is unconstitutional.

Assistant Commonwealth's Attorney Richard F. MacDowell said that prosecutors were unwilling to compromise.

"We've got to consider the challenge to the statute on constitutional grounds, but we're still looking to close the whole thing down," he said.

Show Place Adult Books, one of two adult bookstores in Fairfax County, has been a source of frequent controversy. The corporation that owns the store has been convicted of selling obscene materials at least 79 times since 1975, according to court documents.

The store owners also are facing pressure from the state attorney general, who has asked the State Corporation Commission to dissolve the firm's charter on the grounds that it "has continued to exceed or abuse the authority conferred on it by law."

Most of the testimony yesterday centered on the more than 100 small projection booths and connecting hallways, which witnesses said serve as gathering places for sexual encounters.

"You would know by the kind of looks they give you, what they wanted," said a 27-year-old Alexandria machinist, who said he engaged in sex with other men in the booths. "They may use a hand gesture to lure you into a room."