In July 1987, a large outbreak of shigellosis occurred among attendees at a mass gathering in a national forest, the annual Rainbow Family Gathering. Sanitation in the campsite was poor, allowing widespread transmission of disease, probably by food, water, and person-to-person spread. The attack rate may have been greater than 50% among the estimated 12,700 attendees. The outbreak was caused by Shigella sonnei, resistant to ampicillin, tetracycline, and trimethoprim-sulfamethoxazole; the organism was of colicin type 9 and contained a 90-kilobase plasmid not found in non-outbreak-related strains. The dispersal of the group resulted in nationwide dissemination of the organism, and outbreaks in three states were linked to transmission from attendees at the Gathering. This outbreak demonstrates the potential for rapid dissemination of disease in such a setting and the necessity for careful planning of mass gatherings.