In a white circus tent heated to 105 degrees, 600 not-quite-naked people contort their bodies into positions you never knew were possible. The men have perfect, rippling muscles. The women (and the majority of students here are female) are long and taut, with fatless stomachs curved just enough to be erotic and breasts that perk cheerfully upward. They sit with their legs tucked behind their heads, bodies arranged like pretzels, then gracefully deploy their arms, hips, hands, and legs to open like Georgia O'Keeffe flowers into variations on the split. The mats beneath them are damp with sweat. Overhead, great white plastic ventilating tubes, 70 feet long and 5 feet wide, pump humidity into the air. The vinyl of the tent drips with condensation, and a locker-room aroma hangs in the air.

I've only just arrived, but this bacchanal of bare flesh has been going on for two months. These men and women have come to the sprawling Town and Country Resort Hotel on the outskirts of San Diego to become certified instructors of Bikram Yoga, the controversial American variant that is performed at extreme temperatures. Each has paid $7,000 in tuition and $3,900 in residence fees (all students must stay at the Town and Country) for nine weeks of study, six days a week. This includes two daily 90-minute yoga sessions, as many as five more hours of posture clinic (where they learn to correct their spine or shoulders in particular asanas, or postures), and evening lessons in anatomy and Hindu philosophy followed by Bollywood movies and Indian soap operas until 2:30 or 3 in the morning. When they leave, they will be certified to teach at one of the 5,000 Bikram Yoga studios worldwide.

That's assuming they're able to execute the demanding series of postures that make up Bikram. Right now, the students are in head-to-knee pose, or dandayamana-janushirasana: From a standing position, lift one leg so that it's at a right angle to your body, keeping your knee locked, then bend your upper body forward toward the lifted leg. Imagine the tableau, the kaleidoscope of slim, strong-hipped, bowed bodies, the scene multiplied by the mirrors lining three of the four walls. Now it's camel pose, or ustrasana: On your knees, hands on your hips, bend back until you grab your heels with your hands, then thrust your chest into the air. Before the session is over, 50 or so students have rolled up their mats and left, overwhelmed. I hear what sounds like the chop-chop-chopping of helicopter blades and realize it's my own heartbeat. The ceiling spins. I roll over, open my eyes, and watch the ballet of it in the mirrors. I see more than I bargained for. Because of the heat, everyone is wearing the smallest, tightest thing they can, and, especially with the sweat, the clothes do not cover so much as exaggerate.

Morning practice is bigger than usual today because this is "Intensive Training Week," when many come for the recertification required to maintain their teaching credentials. Most are working through the 84-posture intensive series, the two-hour-plus advanced routine practiced by the elite. This is the portion of the program that is personally supervised by Bikram Choudhury, the 64-year-old founder of Bikram Yoga. Only the best, bravest, and most beautiful practice at the feet of the guru, who sits cross-legged on a giant inflatable leather throne against the back wall. He's in a black Speedo, bare-chested, his hair tied in a topknot. His triceps stand out like pistons. Sometimes a woman will brush his hair or wash and massage his feet. He resembles a cartoon genie on his magic carpet. Between cell-phone calls, he barks Bengali-inflected criticisms and corrections into his headset. He speaks only in exclamation points.

"You, Miss Teeny-Weeny Bikini! Spread your legs! You, Mr. Masturbation! Until I say 'Change,' you do not move a muscle!"

It's hard to tell if these directives are intended for anyone in particular or if Choudhury is just working the crowd. He keeps up a patter of bawdy, sexually suggestive, often male-bashing banter throughout the session. Students—men, especially—have been known to complain, but for most, Bikram's commentary is part of the package. He's built his business, which has been estimated to earn him nearly $5 million a year, in large part by applying a veneer of eroticism to this ancient spiritual practice. For the women here, the "boss," as he calls himself (and everyone else), offers a path to sexual awakening. For the men, Bikram Yoga is a great workout, and maybe an opportunity to get close to a few kundalini-stimulated hard bodies once class lets out.

Choudhury hums "Killing Me Softly" into the mic of his headset as his pupils struggle to hold a posture, even the strongest among them trembling. At last he gives the signal to change.

"This posture called dirty old bitch! Because not even one more inch can you stretch!"

UNDER THE BIG TOP: Choudhury strikes a favorite non-yoga pose.

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Born in Kolkata, India, Where Yoga is a competitive sport as well as a spiritual practice, Bikram Choudhury claims to have become the All-India National Yoga Champion at the age of 13. He left in 1970 after his guru, Bishnu Ghosh (the younger brother of Paramahansa Yogananda, who is generally credited with bringing yoga to the West), instructed him to spread the practice throughout the world. Choudhury's principal innovation—heat—is supposed to increase flexibility and prevent injury, but he came to it by necessity. In Japan, where he first taught, he found himself shivering through his postures during winter, so he brought in space heaters. Suddenly it was easier for his students to lock their knees and touch their palms to the floor. As an added benefit, the saunalike temperatures heightened their sense of euphoria and purification after workouts. In 1972, when he launched Bikram's Yoga College of India in a tiny studio in the North Beach section of San Francisco, the heaters came with him. Bikram Yoga was born.