Approximately 41 percent of the visitors and employees who were at a ski resort in Montana between December 23, 1974, and January 17, 1975, became ill with diarrhea and other gastrointestinal symptoms. The association between illness and water consumption proved to be significant, and the previously noncontaminated water supply was found to contain coliforms. No pathogens except Yersinia enterocolitica were isolated, and it was isolated from two wells serving the area. The role of Y. enterocolitica in this outbreak is not clear, but it was the only organism isolated that is known to be pathogenic.