Contaminated mud ingested by mountain bikers in a race was the likely cause of an outbreak of Campylobacter jejuni in British Columbia. More than 200 of 785 participants became ill. Bears, horses, chickens, and dogs had all used the bike trails prior to the race. Only bottled water in paper cups was distributed to bikers along the course and was not associated with illness. Refilling personal water containers at stations was also not associated with illness. Racers who ate mud had more than two times the risk of developing illness. Mud and water samples were tested for Campylobacter, generic E. coli, and total coliforms. No Campylobacter was found, but mud samples contained E. coli and had high coliform counts. The mud was sampled three weeks after the race event.