One of America's most vigorous real estate booms is underway here on this 32-mile-square speck in the Caribbean.

Two hours by jet from Miami and less than four hours from Washington, St. Thomas is being hit with enormous waves from the mainland of tourists, condominium development and commercial investment.

Egaer visitors are signing contracts for vacation condominum units six months before the foundations are dug, at prices that often range above $100,000 for two- and three-bedroom apartments, and developers are responding with newer and more elaborate projects.

Hotels that had trouble filling rooms in the mid-1970s are now overbooked -- at $80 to $100 or more a day -- and are building condominiums astride their properties for future sale.

The downtown harbor district of the territorial capital city, Charlotte Amalie, which went limp during the 1975 and 1976 recession years, has bounced back with a vengeance. Traffic jams and overdoses of cruise ship tourists are the main problems now, rather than low sales in the duty-free shops.

Two years from now, when the Harry S Truman Airport at the western end of town is lengthened to 7,000 feet to accommodate jets larger than 727s, the flow of people and money into this mountainous, tropical community is expected to accelerate further. As it is, nearly 1 million travelers arrived on this island of 50,000 by cruise ship, amphibious plane and jet last year.

St. Thomas is so upbeat in real estate terms that the Virgin Islands government is worried about the island's long-term "carrying capacity" and the need to control growth with federally funded techniques of coastal zone management.

The government also worries about the growing gap between the wealth of visitors and natives and the contrast between development here and on St. Croix, 40 miles to the south. St. Croix is more than twice the size of St. Thomas and has industrial development in the from of a huge Hess oil refinery, a Martin Marietta plant and other major firms, but is experiencing nothing like the building boom underway underway on St. Thomas. St. Croix is flat, poorer and was the scene of racial violence in the first half of the decade. Like St. Thomas, it also has a low-income emigrant population from other Caribbean islands.

St. John, the third principal member of the Virgin Islands group that the U.S. acquired from Denmark in 1917, is dominated by national parklands and has a handful of resorts and a sleepy harbor village. It shares in the prosperity of its near neighbor, St. Thomas, but studiously avoids the bustle and glitter.

"The problems we've got mainly have to do with balance," says Judith Weiss, a transplanted Arlington resident who is now a senior policy analyst for the Virgin Islands Commerce Department. "We need growth and investment, more housing and real estate development, but we don't want to turn St. Thomas into another Honolulu, or neglect the potential of St. Croix.

"And we don't want to lose the essential character of the islands," through high-rise construction or excessive building on fragile beach areas, she said. With as many as 2,000 additional condominium units on the drawing boards or going into the ground on St. Thomas alone, Weiss warned, "there's a real danger that growth could get out of hand."

Another transplanted Washingtonian, Samuel Schattner, Former owner of the Watergate restaurant and currently the developer of the Watergate Villas condominium project several miles outside of Charlotte Amalie, a grees with Weiss. Schattner, who first began building his units on a 16-acre, rocky bluff overlooking Bolongo Bay in 1971, says St. Thomas is at the same point that Florida was several decades ago.

"There's still an incredible amount of development to come," he said, "but we've got to take great care that we don't go too fast, or harm the environment that attracted us here in the first place."

As it is, development on the island is an engineering and ecologial challenge because of the steep terrain, chronic water shortage and few indigenous building materials -- including sand.

"You've got to think about real estate in very different terms when you're on an island in the middle of the Caribbean," Schattner said. "You've got to think about whether you're going to have electricity or drinking water, or an adequate field for a septic system -- things you tend to worry about a lot less on the mainland."

Schattner's low-rise development, which currently has 170 units and will grow to wupwards of 240 in coming years, is an attempt to congorm to the natural outlines of the terrain. About two miles away, however, a multi-story Holiday Inn rises up at Frenchman's Reef, a cliff at the entrance to St. Thomas harbor.

It looks every bit like the Miami-style development that Weiss, Schattner and other islanders want to avoid.