SOUTH KINGSTOWN, R.I. -- A haze masks Moonstone Beach in the early summer afternoon. Braving a cool breeze, 75 would-be sun worshippers stake out sections of sand, preparing to practice a town tradition: bathing in the buff.

This pristine swath of shore and marsh in the Trustom Pond National Wildlife Preserve is one of the last New England habitats for the clothing-optional set.

And, in a world where scarcity gets more plentiful every day, it is also one of a dwindling number of habitats for the piping plover, a small, skittish shore bird facing extinction on the Atlantic coast.

Federal authorities charged with protecting threatened species -- not including the American nudist -- have proposed closing the 1 1/2-mile beach each year from April through August so that two pairs of nesting piping plovers may multiply and be fruitful. The bare bathers believe the government's real aim is to wipe out New England's "free beaches."

"I think some of this is discrimination," said Joseph Di Pippo, 46, vice president and founder of the New England Naturist Association. "It's people who are fearful of us because we're comfortable enough with ourselves to take our clothes off."

Charlie Blaire, refuge manager, explains it differently. "This is not an anti-nudity proposal," he said. "Congress requires us to put wildlife first. We have no choice."

One evening last week, about 300 people jammed the auditorium of the Matunuck Elementary School for a raucous hearing before the U.S. Fish and Wildlife Service. Playing to a flock of television cameras, they hooted down friends of the piping plover and opponents of public undress. They threatened to take their case to federal court.

Di Pippo, deeply tanned, received cheers and applause after promising legal action, then led camera crews outside the auditorium to explain his frustration with the government's proposal. "If you protect every bird in the world, eventually we will be the endangered specie," he said. "How much protection do four birds need?"

A lot, according to Richard W. Dyer, a biologist with the Fish and Wildlife Service and head of the agency's Alantic Coast piping plover recovery team. "There are more bald eagles nesting in the eastern United States than there are piping plovers," he said.

About 600 pairs of bald eagles, the nation's symbolic bird, are known to be nesting in the eastern United States after an aggressive program to protect the birds and encourage their procreation. They are still on the government's endangered list in most parts of the country.

A sparrow-sized bird with feathers the color of dry beach sand, the piping plover was hunted nearly to extinction at the turn of the century when its delicate plumage was used for women's hats. After federal legislation was enacted in 1918 to protecting migratory birds, its population recovered.

But increasing development of beach-front property since World War II has left only 564 pairs nesting at about 175 sites from Maine to the Carolinas, where they are classified as a threatened species. Elsewhere, they are considered endangered.

The number of New England beaches where nudity is tolerated is also dropping. In 1975, a federal appeals court upheld a National Park Service ban on nudity at its Cape Cod beaches, although a class-action suit aimed at allowing toplessness is pending. Last summer, nudists also lost Massachusetts' Plum Island, part of the Parker River Wildlife Refuge north of Boston.

Experts in such things don't know the precise number of "free beaches," in part because naturists gather to act naturally wherever they believe they can get away with it. The significance of Moonstone, they say, is that it is one of the last beaches in New England where authorities don't prohibit it. Reached at his office in Oshkosh, Wis., Lee Baxandall, author of World Guide to Nude Beaches and Recreation, said, "In general, the number of sites is declining."

At Moonstone, only a weathered log separates a portion of beach managed by the town from the stretch of sand used by nudists. If Moonstone is closed as proposed, beginning in the spring of 1988, bathers who favor swimsuits would be able to use another beach purchased by the town. Those who don't would be left without a place in the sun for the first time in local memory.

"I know people 80 years old who came down here without clothes when they were children," said Charles Mathews, a retired fisher who was raised in South Kingstown.

At least one public official who frequents the area has declined to take sides in the dispute between nudists and advocates of the piping plover. Sen. John H. Chafee (R-R.I.), who with family members owns a beach-front home a few hundred feet from Moonstone, is taking a "hands-off" position, press secretary Cleve Corlett said. Chafee, however, adopts a suits-on policy when he walks the beach.

The suits-off crowd is preparing to make a last stand. Leo Levesque, a 66-year-old clad wholly in a pair of wire-rimmed glasses, faces the cool, onshore breeze and says efforts to protect the piping plover are a ruse to end nudity on federal land.

The proof, he said, is that he seldom encounters camera-toting tourists in pursuit of protected wildlife. "They come for the nudes," he said.