Welcome to the end of the road. If you're going to Sian Ka'an Biosphere Reserve, it stops here, near the tip of a long, skinny peninsula about 100 miles south of Cancun. Scrub jungle encroaches on the village, fishermen wait for the opening of lobster season, the town drunk sleeps it off, dogs bark, flies swarm, flotsam surfaces on the beach and bread rises in the panaderia.

That's about it for Punta Allen, which promises nothing to visitors seeking parasailing, shopping, golf and other Cancun-style diversions. Development, which has crept down the Yucatan coast during the last 10 years and turned stretches of wild beach into a self-styled Maya Riviera, hasn't yet reached Punta Allen. And it isn't likely to soon because the construction of tourist facilities is strictly limited in the roughly 1.5 million-acre ecological reserve that surrounds the drowsy Mexican village.

If, however, you lust to catch a bonefish with a fly and rod, Punta Allen may be one of the most exciting places on earth.

The bonefish, Albula vulpes, is a creature only a fly fisherman could love--small (3 to 8 pounds in this area) and too bony to eat, which is why they're generally pursued on a catch-and-release basis. They populate the shallow, mangrove-fringed flats of Sian Ka'an's 20-square-mile Ascension Bay. The balmy habitat makes an appealing change of scenery for fly fishermen used to braving the elements on North American trout streams, but it gives the fish a critical advantage: They are almost impossible to see without the help of expensive polarized sunglasses and the trained eyes of a local guide. And even if your casting is precise, the tricky devils are strong and fast, as people who hook and then lose them are dismayed to discover.

"It's the ultimate trout fishing," says the owner/namesake of Bob Marriott's Fly Fishing Store in Fullerton. The travel department at Marriott's store sends anglers to several small resorts in the area between Sian Ka'an and the Belize border. It is one of the best places in the world to fish for bones, together with the Bahamas, Los Roques in Venezuela, the Seychelles and Christmas Island.

Before heading to Sian Ka'an last fall, I took casting lessons with Kevin Bell, a fly-fishing instructor and general manager at Marriott's store. By the pond in a Fullerton park, he showed me how to keep a tight loop in the line and apply power at the end of a cast. It's harder than it looks, a completely clumsy enterprise for a novice, until you get the hang of it. By the end of two lessons, I was making reasonably well-formed casts. Of course, I still had no idea of the complex tactics required to hook and land a bonefish. Nevertheless, my teacher seemed upbeat about my chances, though I could tell he thought it strange for a rookie to go after bones, which, I guess, is like someone who's just learned how to ride a bike entering the Tour de France.

The truth is, I just wanted an excuse to visit Punta Allen, which, on a visit to the Yucatan peninsula 10 years ago, had lodged in my memory as one of the world's great end-of-the-road places. I had taken a VW Bug on the perilously potholed, unpaved 30-mile road that scratches its way down the peninsula, often within sight of both the Caribbean to the east and Ascension Bay to the west, but had turned back when the going got too rough.

The average Cancun vacationer never catches wind of Sian Ka'an and the lost world of jungle, beach and reef that lies between the Maya coastal ruins of Tulum, about 60 miles south of the international airport, and the Mexico-Belize border. Then, too, the infamous Boca Paila Road to Punta Allen culls out all but the most determined.

"Some people make it some of the time," Manuel Sabido, the office manager of Cuzan Guest House, where I stayed in Punta Allen, told one of the guests. Everyone at the table laughed uproariously, but all the new arrivals had bruised behinds and glazed looks from the drive.

Knowing the road's rigors, I had Sabido book me a transfer to Punta Allen from the airport, where I was met by a beat-up white Chevy Suburban. Pedro, the driver, and his blase young sidekick, Luis, looked as though they could handle anything, so I kicked back and watched the scenery along Highway 307, which follows the east coast of the Yucatan peninsula all the way to Belize.

The last time I drove the road, nothing interrupted the low, thick jungle, though the area south of Cancun had just been designated a development zone by the Mexican government. Now there were resort entrances every few miles, all grandiose faux-Maya, attracting Europeans for sun-and-sand package vacations. In Playa del Carmen, I saw new fast-food outlets and shopping malls. Formerly a sleepy village with little more than ferry service to Cozumel, it is now one of the fastest-growing cities in the world.

We passed through Tulum, then stopped at the reserve entrance to pay for admission, about $2 a day. Hotels and houses, ruined by one hurricane or another, sat moldering beside the road. Soldiers assigned to drug smuggling-watch along the beach, wearing hot boots and uniforms, hitchhiked along the extenuated peninsula. It had rained recently, turning potholes into swimming pools filled with standing water high enough to reach the Suburban's door handles. Every time we launched through one, Pedro grinned determinedly and Luis laughed. "I like this," he said. "It's an adventure."

About 10 miles into the reserve we crossed a wooden bridge over the cut near Boca Paila Fishing Lodge, where Pedro pointed out a manta ray undulating by a stanchion and a roseate spoonbill stalking fish on a flat. The lodge there is one of the more upscale of about half a dozen around Ascension Bay, and the place where bonefishing was introduced to the area about a decade ago, just as lucrative spiny lobster harvests seemed to be falling off.

Once the "gray ghosts"--as bonefish became known--were discovered here, North American fly-fishing experts came south to teach Punta Allen lobstermen and lodge guides the tricks of the trade. Locals had to learn how to cast and tie flies, and at first thought the whole catch-and-release enterprise crazy, says Sonja Lillvik, who opened Cuzan around 1985 with her partner, Armando Lopez. Now, guiding bone fishermen in the winter and spring is an important source of income for the people of Punta Allen, besides summer harvesting of lobsters, which are coming back.

There, on the tip of a fish hook, is what Sian Ka'an--and other biosphere reserves selected by the United Nations Educational, Scientific and Cultural Organization--is about: balancing preservation of some of the world's rarest, most biologically diverse places with the need of indigenous populations to survive and prosper.

Sian Ka'an was decreed in 1986 and enlarged in 1994, and now is operated by the Mexican federal parks system, with extensive help from the Amigos de Sian Ka'an, a not-for-profit group created by environmentalist Barbara MacKinnon de Montes, who was born and raised in the U.S. but is a naturalized Mexican citizen. Though pressure to develop the reserve has been mounting, they have instituted a slow-growth plan for Sian Ka'an, emphasizing low-impact eco-tourism and limiting the number of hotel rooms to 1,500. "I would prefer not to see any development," MacKinnon says. "But the area is a biosphere reserve and thus 'sustainable' development is an integral part of the management plan."

To that end, MacKinnon's group and other conservationists encourage traditional cottage industries such as honey harvesting and hammock weaving. They support reforestation and crocodile monitoring efforts. And, perhaps most important, they teach some of the more than 800 residents of Sian Ka'an how to serve as eco-tourist guides. Catch-and-release fly-fishing fits nicely in the program.

Thankfully, the reserve is, as yet, too rough and off-the-beaten-track to be in imminent danger of overdevelopment. A third of it is tangled mangrove islets, swamps and mirror-like flats. Another third is Caribbean coast, looking out to the long reef that borders the Yucatan. The rest is machete-dulling tropical jungle, virtually untrammeled since the time of the Mayans, who left 23 ruins and a streak of proud independence to their descendants in the area.

Bumping around in the back seat, I tried to read a guidebook about the fauna of Sian Ka'an: crocodiles, jaguars, snakes, manatees, tapirs, pumas, leatherback sea turtles, howler monkeys, almost 250 species of birds--including jabiru storks, parrots, toucans, egrets and flamingos--and all the fish of the bay, reef and deep. Suddenly the Suburban came to a stop. We were finally at Cuzan, though by now it was too dark to see anything beyond a large, round, palapa-roofed restaurant, where lights glowed and the sand floor invited bare feet.