The conservancy plans to create a nature preserve for marine and climate research and limited tourism that would be sensitive to the ecology.

The precise purchase price was not disclosed, but officials of the Nature Conservancy say they must raise $37 million to buy the atoll and establish operations and an endowment, making the atoll the single most expensive acquisition in the 49-year history of the nonprofit conservancy, which is dedicated to preserving wild areas around the world.

Palmyra has five times as many coral species as the Florida Keys and three times as many as Hawaii. It is home to the world's largest land invertebrate, the rare coconut crab, and a population of red-footed booby birds second only to that of the Galapagos Islands.

"It is the last marine wilderness area left in the U.S. tropics," said Chuck Cook, the conservancy's project director for Palmyra. "The fact that there are no human pressures to speak of is unique and the opportunity to protect this pristine reef is fantastic. It is one of the few nesting areas for seabirds in a 450,000-square-mile area."

The conservancy has already raised about $10 million, Mr. Cook said, and hopes to raise an additional $8 million by selling part of the area to the United States Interior Department for use as a federal fish and wildlife refuge.

With 15 feet of rain a year, yet as much sunshine as Hawaii, Palmyra is a rare, "wet" atoll able to sustain life and vegetation, including one of the last remaining undisturbed stands of Pisonia grandis beach forest in the world.

Over the years, Palmyra has been a stopping point for Pacific yachtsmen, a United States Navy supply base in World War II, the site of a proposed nuclear waste dump, of an unsuccessful coconut plantation and of various development schemes that never got off the ground. Two years ago it attracted the attention of Microsoft's Bill Gates, who considered buying it as his private getaway.

But it is perhaps most famous as the scene of a 1974 slaying in which a married couple were killed and their boat stolen by fellow yachtsmen whose own craft was sinking. The case became the subject of a best-selling thriller, "And the Sea Will Tell," by Vincent Bugliosi, the lawyer who won an acquittal for one of the defendants.

Palmyra, situated about 300 miles north of the Equator and two hours by large jet from Honolulu, was named by an American ship captain whose vessel, the Palmyra, blew ashore in a storm in 1802. In 1816, a Spanish pirate ship, the Esperanza, loaded with plunder from Incan temples, was wrecked on the atoll's coral reefs and its cargo never recovered. In 1862, two New Zealanders who had married Hawaiian women obtained a deed to the atoll from King Kamehameha V, and when Hawaii was annexed to the United States in 1898, Palmyra was specifically excluded. It later passed to other private owners.

In 1922, a family of Honolulu roofing contractors, the Fullard-Leos, bought the atoll for $70,000.

Beginning in 1938, the Fullard-Leo family embarked on a long-running feud with the Navy, which built an airstrip on the atoll and used it as a base in World War II. After the war, when the government tried to keep the atoll, the Fullard-Leos waged a successful fight to retain title in a case that went all the way to the United States Supreme Court. When Hawaii became a state in 1959, Palmyra was again excluded.

Three surviving Fullard-Leo brothers, Leslie, Ainsley and Dudley, control the property. All are now in their 80's or 90's, and repeated efforts to develop the property have either failed or been rejected by the family as too disruptive to the atoll's ecology. The Nature Conservancy has been negotiating to buy the parcel, which includes 680 acres of land and 15,512 acres of reefs, for more than two years, at a price well below the $47 million the family initially sought, with the family taking a tax deduction.

"Potential buyers all had plans for much larger developments than the family was comfortable with," said Peter Savio, the Honolulu real estate agent who represented the Fullard-Leos and who himself had made an unsuccessful effort to develop the the atoll as a resort. "The family is very happy at the sale to the Nature Conservancy. It's the kind of use they've always looked for."

The conservancy has until the first quarter of next year to raise the rest of the money.

The United States Fish and Wildlife Service identified Palmyra last year as its No. 1 land-purchase priority, and $8 million in federal money has already been appropriated toward the purchase, subject to the conservancy's ability to acquire the atoll.

Mr. Cook of the conservancy said his organization was in the process of building a small tent camp, improving the overgrown runway, with the expectation of eventually teaming up with a tour operator that could allow 30 to 40 people at a time to visit the atoll for scuba diving, snorkeling and catch-and-release bonefishing, at a cost of $3,000 to $4,000 a week. But, Mr. Cook said, "Eco-tourism is not our primary objective; preserving these robust and abundant habitats is." Error: Unable to read footer file.