Deep in the high-rise canyons of Rosslyn, near apartment towers where the rents are expensive and the door knobs ornate, Lois Elliott, 63, recently spotted an X-rated movie.

Stunned, she said she wondered how such a thing could exist in her "rather high-class neighborhood." She immediately organized a protest.

Her alarm has spread, enlisting the signatures of 200 office workers and tenants in the Rosslyn Plaza, Normandy and London House complex, Yesterday, Arlington County Board Chairman Dorothy Grotos joined the protest, asking county officials if they could force the theater to stop showing the X-rated films.

Commonwealth Attorney William S. Burroughs, six weeks ago dispatched vice squad detectives to the Rosslyn Plaza Theater to warn the manager that "hardcore was outside the limit of what Arlington County could live with," said Lt. Walter Hughes of the vice squad.

Theater manager Andrew Denney said the theater has since changed the movies from hardcore to softcore. Yesterday was the last day for a softcore version of "Misty Beethoven," a film which, in its hardcore version, set the record for the longest continuously playing movie in Washington.

Paul Kerchner, the manager of the K-B Theater chain, which runs the Rosslyn Plaza Theater and 12 others in the Washington area, said the crowds that come to the softcore films are "well-dressed, well-reined, very sophisticated. We don't get dirty old men in overcoats."

Elliott, who started the Rosslyn protest, disagrees. "Just last week I saw a real crummy looking man -- it looked like he just got off Metro -- to into the theater. We don't need this," she said.

Board Chairman Grotos said she doesn't care whether movies shown in Rosslyn are called soft or hardcore. "It is still X-rated," she said.

If the county officials determine that films cannot be regulated under county obscenity laws, Grotos said she will seek an ordinance ot forbid or limit the showing of X-rated films in Arlington.

There is only one other X-rated movie house in the county, the Byrd Theater, and it is located in the Washington-Lee Shopping Center near Rte. 50. "But that's across from the Army base," Grotos said. "This theater in Rosslyn, it is not image I hoped to get established there."

The county has spent nearly $9 million in an effort to revitalize once-decrepit Rosslyn, an area where high-rise construction has caused continuing controversy. Some apartment dwellers in Rosslyn, who have grown to like the tall buildings, affectionately refer to it as "Little Manhattan."

Theater manager Denney said he chose the X-rated movie format in January when he reopened the theater after it had been closed for two years.

"This is a hard theater to find," Denney said, "so we have to attract the business people that are already here." He said he's hoped that the nearby Metro subway station would help his business, but that so far his profits have been "nothing to write home about."

Next door to the theater, at Fredy's of Rosslyn beauty salon, Fredy Bayerle said he understands the X-rated tempest that has arisen in the neighborhood among "ladies of the leisure class."

"They worry about what kind of persons these films are oing to bring in," said Bayerle.

Elizabeth Young, one of the organizers of the protest, said that "all we want to do is keep the quality here."

Young said she has suggested to the protesters that "one of us go and see the films. You know the ones I mean, 'Inside Somebody' or ' Jennifer Beethoven.'"

Sitting in a beige wing chair on the sixth floor of the Normandy building, with a view of the Potomac River and Roosevelt Island, Young said she thought Arlington County was trying to build up the quality of Rosslyn.

"I remember the old days," Young said, when Rosslyn was a conglomeration of pawn shops, lumber yards and railroad tracks. "Now with this (X-rated theater), Rosslyn might go back full circle to the way it was," Young said.