Unspoiled? In the Caribbean?

VIEQUES ISLAND, Puerto Rico - Swimming in the warm, bioluminescent waters of Vieques' Mosquito Bay, there are more "stars" at your fingertips than there are in the inky sky overhead.

Tiny, twinkling specks trickle from your cupped palms and cling to your clothes like living sparks, gleaming for a moment before fading away. Under the surface, your arms and legs create a luminescent green wake as you paddle, making you feel like a human glow-worm or a firefly looking for some action, thanks to the presence in the water of billions of microscopic organisms known as Pyrodinium bahamense that light up when disturbed.

"Now this was worth the price of admission," says Paul Leveillee, a Massachusetts-based mortgage broker honeymooning with his wife, Wendy, as he fans his limbs to make a ghostly angel.

It's a trippy, otherworldly encounter - and Vieques, a narrow, 21-mile-long island off the coast of Puerto Rico, is one of the few places in the world where you can experience this. Not surprisingly, the bio-bay is the top attraction on an island that very few sun-seekers have heard of and that is one of the last relatively undeveloped outposts of the Caribbean.

The U.S. Navy, which used Vieques as a bombing range for 60 years, is largely responsible for maintaining the island's low profile. The armed forces moved in during the early days of World War II, erecting chain link fences topped with razor wire that rendered two-thirds of the island off-limits. Though the Navy shelled only a small portion of Vieques a few times a year, travel agents apparently didn't consider a live-fire training facility a prime vacation destination. So for decades, only adventurous tourists ventured here to discover the sugary white beaches lapped by clear aquamarine waters, and the laid-back lifestyle of the 9,000 or so primarily Spanish-speaking residents who occupied the remaining one-third of the island.

In 2003, following years of protests against the Navy's presence, the boys in blue withdrew. Now Vieques has become a buzzword among travelers looking for the next "undiscovered" frontier.

Most of the Navy's former holdings have been turned over to the U.S. Fish and Wildlife Service, meaning that the majority of the island will remain undeveloped. This translates to thousands of acres of undulating hills where horses roam free and miles of beaches with no buildings of any kind, save for a few small picnic gazebos nestled beneath the palms.

Some areas remain fenced off, but most of the island can be explored, provided you're armed with four-wheel drive (and a good support bra) to negotiate the bumpy, unpaved roads that snake through the wilds and lead to several former Navy beaches. But any who brave those dotted lines on the map may well find themselves the only souls on the beach, except for the occasional topless sunbather (illegal, though not uncommon) or a fisherman swinging his fishing line overhead like a lasso as he wades into the surf.

There are only two towns on the island - Isabel Segunda, an Atlantic port, and Esperanza, on the Caribbean. Isabel Segundo is larger and more populous, with a maze of restaurants and shops sandwiched between modest cinderblock homes. The main tourist attraction: a 150-year-old former Spanish fort that now serves as a museum. A few miles west lies Vieques' only large hotel, the 138-room oceanfront Martineau Bay Resort & Spa, which is to become a W Hotel in fall of 2006.

Esperanza, by contrast, is a breezy little beach town with almost no commercial development beyond a half dozen open-air restaurants and bars along the Malecon, an attractive beachfront boardwalk that serves as the heart of the community. On weekends, tourists and locals mingle over beers and burgers at Bananas (motto: "A Gin-U-Wine Sleazy Waterfront Dive"). They groove to tunes by Blondie and Michael Jackson at La Sirena, a seafood restaurant that occasionally doubles as a dance club. And they wait for the tables nearest the street at Bili's, a seafood-and-steak restaurant, in order to watch the action unfolding along the Malecon.

In the evening, young men and women canter spirited horses up and down the road, vying for space with the cars that slowly cruise by blaring Latin music. A local family plays dominos on the boardwalk, and behind La Nasa, a tiny waterfront cantina, middle-aged couples sway to a symphony of salsa and surf.

During the day, families play in the waters just off Esperanza's municipal beach. Children jump off the pier, and Randy Edwards, a bartender who moved here five years ago from Marietta, Ga., sits in a plastic chair outside La Nasa smoking a cigar.

"It's just another lazy Sunday afternoon," says Edwards, who serves his signature Caribbean Cosmopolitan (a combination of Bacardi Limon, white cranberry juice and cointreau) at MBar, a hip watering hole and restaurant located across from Martineau Bay. "I'll have a cigar, a couple of beers, and go swimming in the afternoon." Beyond these simple pleasures - swimming, snorkeling, fishing, hiking, horseback riding, mountain biking - there's not a whole lot to do on Vieques. According to your point of view, that's either a blessing or a bane.

"Vieques is not for everyone," admits Rebecca Wilmot, whose small bakery, La Dulce Esperanza, is located in a neighborhood of tidy houses just uphill from the Malecon. "If you like casinos and shows... " Wilmot's voice trails off and she shrugs, meaning "Sorry, buddy, you're out of luck." There's not even a local movie theater.

But Vieques has come a long way since Wilmot's first visit in 1984, a few years after her father bought a bat-infested restaurant in Esperanza. "The airport looked like a chicken coop," she recalls. "The Malecon wasn't built yet, and there was trash everywhere." Twenty-one years later, that restaurant, Tradewinds, is one of Esperanza's most popular eateries, and property values have gone through the roof.

"The year the Navy left, it was like the Gold Rush," recalls Wilmot, a former nurse from Boston who moved here four years ago. "Everybody was coming down and buying property."

Growth isn't necessarily a bad thing, according to Hector and Mary Matos, who run Hector's by the Sea, a collection of modest guest cottages overlooking the Caribbean near Esperanza.

"Tourism is rising little by little, but there's not enough yet for tourists to do, like shopping and restaurants," says Hector, a native of Puerto Rico who grew up on Long Island, N.Y. "And you don't have enough rooms. I think a couple more hotels would be just fine."

Mary, who lives with Hector and a bilingual parrot named Frankie in a one-room cottage tucked among the guest houses, nods in agreement. "We have to have progress, but it has to be controlled," she says, her gaze wandering to the horses grazing beside her pool. "It has to be responsible, with good planning and people who care."

Delicately poised as it is between backwater anonymity and burgeoning recognition, it's not surprising that Vieques has become something of a celebrity retreat. Jennifer Lopez and her husband, Marc Anthony, are rumored to own land on the island. Uma Thurman and Will Smith have reportedly vacationed here, and Richard Gere stayed at the Martineau Bay Resort & Spa while on a break from a movie set on mainland Puerto Rico.

That's all fine for the paparazzi who, fortunately, do not seem to have discovered Vieques. But the most impressive stars are those you'll find in the glittering universe of Mosquito Bay.

Winding Down on Vieques

Getting there: Fly into San Juan International Airport. From there, you can fly into Vieques or take a ferry from Fajardo to Vieques. For flight information, visit http://www.vieques-island.com/geninfo.shtml. For ferry information, call 787-741-0800.

Lodging: Martineau Bay Resort & Spa has 138 spacious guest rooms and suites, two Atlantic Coast beaches, tennis courts, a spa and a pool with a swim-up bar. In fall 2006, the luxury gated resort is to become a W Hotel. $309 to $899. 787-741-4100. www.martineaubayresort.com.

The Inn on the Blue Horizon, located about one mile west of Esperanza, offers casually elegant accommodations atop a bluff overlooking the Caribbean. One-story duplexes, some with ocean views, have a total of 10 rooms. $110 to $370. 787-741-3318. www.innonthebluehorizon.com.

Hector's by the Sea, just west of the Inn on the Blue Horizon, has modest guest cottages around a pool and Jacuzzi with panoramic views of the Caribbean. Each of the three rooms has its own hammock, kitchenette and bathroom. $85 to $165. 787-741-1178. e-mail: HMBTC@HOTMAIL.COM.

Dining: In addition to the watering holes along the Malecon, check out Island Steakhouse on Route 201, a romantic restaurant tucked in a canopy of papaya, mango and coconut trees. 787-741-0011.

La Dulce Esperanza offers one of the best breakfast or lunch deals around, with ham and cheese sandwiches on fresh-baked bread for $2.50; large pizza $15. 787-741-0085.

Mamasonga is a tiny breakfast joint in Isabel Segunda that serves heaping portions of French toast, pancakes and omelets. 787-741-0103.

Al's Mar Azul, across the street from Mamasonga, has pool tables, dart boards and tables right on the water. Drinks only; no food served. 787-741-3400.

Getting around: A four-wheel-drive vehicle is recommended. For a listing of rental agencies, visit www.vieques-island.com/geninfo.shtml#car.

Touring the bio-bay: Island Adventures uses an electric boat to ferry guests onto Mosquito Bay. $30 adults, $15 children 4-12, free ages 3 and under. 787-741-0720. www.biobay.com.

Blue Caribe Kayaks takes guests on a guided kayak tour of Mosquito Bay. $30 per person. 787-741-2522.

For more information: Puerto Rico Tourism Co., 1-800-866-7827, www.gotopuertorico.com. Also check out these Web sites: www.vieques-island.com. www.enchanted-isle.com. www.viequestourism.com.