THIS IS A CINDERELLA story. Girl grinds it out as a welder in a large Canadian city. Girl rides a mountain bike extremely well. Girl enters a downhill race and cleans even the pros' clocks. Girl wins the national championship in her first year of racing and wears the maple leaf jersey at the world championships, where, despite a thrown chain, she has the best run of the Canadian women. But the fairy tale ends there.

The fastest girl in Canada used to be a man.

Since 33-year-old Michelle Dumaresq burst onto the scene, in May 2001, winning the novice class at the Bear Mountain race, in Mission, British Columbia, by 2.5 seconds over the fastest female pro, she's kicked up a dust storm of gender-bending gossip, debate, and polemics. And to spice up the controversy, the very people who ushered her into the sport are the ones clamoring to get her kicked out.

Dumaresq was discovered in 1999, three years after her sex-change operation. She was sticking five- and six-foot drops down the Pink Starfish trail, on Grouse Mountain—on Vancouver's North Shore, where she'd ridden since she was a boy—when a dozen top women racers arrived to shoot a video. Wowed by her out-of-nowhere moves, they cast Dumaresq in the 2001 bike-chick flick Dirt Divas and encouraged her to race.

Dumaresq was up front about her transsexual history, and her new friends were cool about it—at least until she beat them. Starting in 2001, she rocketed up the Canada Cup Mountain Bike Series from beginner to expert to pro, winning the national series in 2002. In 2003 she dominated again. Though she endoed during the Canadian National Mountain Bike Downhill Championship, held in Whistler last July, Dumaresq still won by a decisive margin of 2.62 seconds.

But her fairy godmothers have been crying foul. In July 2001, bowing to pressure from racers and anxiety over the potential sex storm, the Canadian Cycling Association and the Switzerland-based Union Cycliste Internationale (UCI), which regulates both the Tour de France and World Cup mountain-bike racing, suspended Dumaresq's license. In April 2002, they reissued it, based on the fact that her official B.C. birth certificate now reads FEMALE, a form of revisionist history at the most personal level.

It was another in a string of international rulings on gender that threaten to turn the sports landscape upside down. As the athletics world wrestles with drugs like steroids and erythropoietin (EPO), the once straightforward concepts of male and female are being challenged by medical science—and by the ambitions of competitors like Michelle Dumaresq.

"Today, we all fight against doping and try to be natural athletes," says 26-year-old French downhiller Anne-Caroline Chausson, who won her seventh world championship in 2003. "Don't we open a door for genetically modified athletes—or worse? Why not clone Carl Lewis to race against Marion Jones?"