A Northern Virginia bookstore, which earlier this year was fined $42,000 for possesssing obscene movies, won reversal of a dozen of its convictions yesterday in a case that revolved around the question of what is legally obscene in Fairfax County.

A Fairfax Circuit Court jury overturned 12 convictions against Croatan Books Inc. of 8653 Richmond Hwy. after viewing 32 films featuring sexual activity and titles like "Sweet Alice and her Friends" and "Leather Lust." a

The jury left standing a lower court ruling that held the remaining 20 films police had seized at the store were obscene. The jury scaled down penalties for the obscenity offenses from the maximum $1,000 fine for each film that the firm had received in its previous trial to an average of $780.

Total fines assessed by the jury yesterday amounted to $15,600, or less than half of the amount imposed on the company by General District Court Judge J. Conrad Waters in February.

The store, one of two adult bookstores that prosecutors say operate in Fairfax County, had appealed its conviction on 42 obscenity charges to circuit court and pleaded guilty to charges on 10 of the films. Fines on those charges will be set at a hearing May 22.

Defense attorney Roger Zuckerman claimed yesterday's acquittals as a victory for his client, although he said the store would appeal the convictions to the Virginia Supreme Court.

"[The acquittals] represent a perception of what is tolerable in Fairfax County that is very much at odds with the perception of Fairfax County prosecutors," Zuckerman said. "The jury takes a different and less archaic view of what is tolerable in a large, cosmopolitan suburban community."

Assistant Commonwealth's Attorney Rodney G. Leffler, who prosecuted the case, said the jury's decision is "no great beacon. . . to guide us in future cases."

He said there was no identifiable difference between the films that drew convictions and those that did not, and theorized that the acquittals were the result of jury fatique. "They were inundated with evidence," he said. "No human being can sit and watch that ridiculous stuff for any amount of time and retain any sensibility about what they're seeing."

To convict an individual or corporation under Virginia's obscenity statute, prosecutors must prove that material violates community standards, using guidelines set by the Supreme Court in 1973. Under those guidelines, material cannot be held obscene unless it appeals to a shameful or morbid interest in sex, goes beyond the customary limits of candor and has no serious literary, artisitc, political or scientific value.

Jurors scurried out of the courtroom quickly at the conclusion of the Croatan case and declined to discuss their criteria for determining which films were obscene. "I think we've been through enough already," said jury foreman Barbara Pirhalla.

According to Leffler, Fairfax prosecutors have established a set of community obscenity standards. "If it shows the act of sex and it shows it without any type of plot, and then goes on to be very explicit in areas of oral, anal, group sex or open ejaculation -- that is what we think is the community standard in Fairfax County," he said.

"It's not like we're prosecuting 'Joy of Sex' or 'Tropic of Cancer'", he said.

Zuckerman maintained that the films were not obscene because they avoided views of sex involving children, rape, violence, incest, animals bondage or other factors that he said appealed to a sick or morbid interest in sex.

The 42 films, each running 5 to 10 minutes long, were confiscated in police raids on the bookstore now called Showplace, on Jan. 24 and March 8, 1979. The officiers, responding to complaints by Mount Vernon area citizens about the one-story cinderblock structure, spent two days viewing the films in 25-cent peep shows in the store's viewing booths before confiscating them.

Area citizens maintained that the bookshop, which prosecutors say has been convicted on up to 100 obscenity counts in its five-year history, jeopardized the morals of their children and attracts undesirable people and businesses to the area.

"We're just trying to protest our moral environment," said Joyce Andres, a Mount Vernon housewife who told the court that she had polled more than 800 acquaintances and neighbors and found none who approved of the bookstore. "There's a lot of pollution around now, and we're worried about our moral environment."