SAN DIEGO (AP) _ A remote strip of pristine sand long labeled America's most popular _ yet unofficial _ nude beach is covering up.

After years of tolerating sunbathers in the buff, the city of San Diego is again trying to enforce a decades-old prohibition against nudity at Black's Beach.

Some who frequent the 1,500-foot stretch of coastline in the wealthy enclave of La Jolla have complained about an increase in sexual behavior and expansion of the nude beach boundaries by the clothing-optional set.

``What's happened is that it's become a place where exhibitionists go,'' said longtime resident Melinda Merryweather. ``You can't take children down there and leave your kids on the beach when you go out in the water.''

The city is seeking to drive nudists north to Torrey Pines State Park, where nudity is still permitted on more then two miles of state-owned beach property.

As of July 1, those who bare all on city property risk a citation and a $135 fine.

``There was a misperception by some that because we turned a blind eye to nudity that they could get away with other activity,'' said Scott Tillson, chief of staff for Councilman Harry Mathis, who represents the La Jolla area.

This isn't a new issue at Black's Beach, named for a founding family of La Jolla but officially known as Torrey Pines Beach. The southern portion of the property near Scripps Pier belongs to the city; the northern part belongs to the state. The dividing line is the city's clifftop park for hang gliders.

In 1974, the city decided to allow nudity at the beach, but voters reversed the ordinance in 1977. Ever since, San Diego's lifeguards have sporadically enforced the ban but generally turned a blind eye to nudists.

The problem isn't unique to Black's Beach.

Officials in Sandy Hook, N.J., are cracking down on nudity at South Gunnison Beach, where reports of lewd acts and sex crimes involving children rose when it was listed on the Internet. Thousands of visitors now visit the once quiet hideaway for naturists, coming from as far away as Connecticut and Maryland.

Gary Hanauer, a freelance writer who compiles an annual guide to California's nude beaches, said nudists fear that the clothing-optional zone will get too crowded.

Black's Beach hasn't been overwhelmed. Those who routinely visit the beach say complaints are exaggerated and a live-and-let-live attitude prevails.

``It doesn't bother me at all,'' said Steve DiPalma, surfing at the beach Monday in a full-body wetsuit. DiPalma, 37, a substitute teacher who has surfed twice a week for more than 10 years at Black's Beach, said he's never seen any lewd conduct.

A topless sunbather agreed.

``I've never been offended by anything that's gone on down here,'' said Mindy Duncan, 28, a San Diego department store manager.

Anti-nudity signs were posted last week, and beachgoers already seem to be getting the message. On a recent afternoon there wasn't a single person au natural on the city section, while about two dozen unclothed sunbathers, mostly men, relaxed on the state-owned portion of the beach.

Ken Wisniewski, a 42-year-old dispatcher for the San Diego Water Department, said he will simply throw on a pair of shorts when he strolls along the city-patrolled section. But he doesn't think he'll have to do it for long.

``It won't last,'' he said of the al fresco crackdown. ``They'll give up after this season. They always do.''