Young Western women, stripped down to bare essentials to catch the tropical sun, pass out candy and coins to begging village children. Male backpackers bargain for some opium and maybe a tribal tryst for the night.

Such cultural collisions are becoming ever more common as once remote Asian regions open up to tourism, bringing worries about the breakdown of vulnerable societies and environmental harm.

In Laos, with rich tribal traditions, pristine landscapes and a fledgling tourist industry, the government and foreign groups are trying to head off such damage while reaping some of tourism's rewards.

Here in the mountains of northern Laos, home to the Akha, Hmong and 36 other officially recognized ethnic groups, trekkers are guided to carefully selected tribal villages that receive $1.30 for each tourist, revenue that is used for medicine, schooling and general community welfare.

The guides, locally recruited and knowledgeable, explain cultural taboos to the visitors, such as not touching Akha villages gates, where guardian spirits are said to dwell. And they interpret the ways of foreigners to often perplexed hosts. Tour groups are limited to a maximum of eight to avoid straining supplies of food in villages visited.

Boontha Chelernsuk, a tourism official, said the Nam Ha Eco-tourism Project has brought other benefits. With tourist income coming in, illegal logging and hunting of wildlife by poor tribesmen have diminished.

Health is improving, too. As part of the training on how to host foreigners, villagers learn about using toilets, boiling water, sleeping under mosquito nets and preserving the environment.

"Nam Ha has become a model. We're going to replicate it in other parts of Laos," said Steven Schipani, an American who was key in launching the award-winning, government-UNESCO project four years ago.

Schipani said the Luang Nam Tha area will be used to train guides and tourism officials from other provinces where similar projects are planned. He hopes private operators will emulate the approach.

"Laos is still very much at the point where we can catch runaway development, but it can go haywire because tourism can be a real money-spinner," he says. "I worry things can go the way of the freewheeling Thailand model, which makes a lot of money."

Tourism became the top foreign income earner for Laos in 2000, when the industry earned $113 million, ranking ahead of hydroelectric power sales, logging and garment exports.

Long isolated by war and communist revolution, Laos was "discovered" in the early 1990s as being among the world's shrinking number of "unspoiled places." Tourist numbers have soared from 37,600 in 1991 to about 700,000 last year.

"My vision is to make Laos the center of eco-tourism in Southeast Asia," said Schipani, a New Yorker who worked as a Peace Corps volunteer in neighboring Thailand. "The decision-makers are starting to understand they can't compete with Thailand in resorts, shopping and nightlife, so they are going the eco route."

In a still largely subsistence economy, community-based tourism brings in cash needed by rural people for basic goods and may keep them from migrating to towns in search of jobs.

"Village women can make more in one hour by cooking for a tourist than collecting bamboo shoots in the forest for a week," Schipani said. "Some of our best guides are former hunters. They can get five dollars a day instead of killing a bird for one dollar."

But even eco-tourism brings challenges.

"It's all changing very quickly," said Bill Tuffin, a longtime Laos resident who started the Boat Landing Guest House, a praised eco-tourism lodge. "Ten years ago you could see the Akha in their traditional costumes everywhere. Now, it's almost all gone except deep inside the countryside."

As happened earlier in tribal areas of northern Thailand, some people are ashamed of wearing traditional attire in front of foreigners or don it just to beg or be photographed for a fee. Village girls may ape Western fashions, like coloring their hair.

"Most tourists don't want to leave a big footprint. They want to have that meaningful kind of experience with local people," said Tuffin, from Pueblo, Colo. "But it's an art to have tourists going into a village again and again and not have them leave a negative impact."

Tourists walk a trail with a Laotian guide near Luang Nam Tha. Tour groups are limited and tribal villages receive a payment for each visitor.