An outbreak of E. coli O157:H7 infections was linked to drinking water from the municipal water supply of Alpine, Wyoming. The water supply of Alpine originated from an underground stream, was collected and stored in a tank, and then distributed, unchlorinated, to the town. There had been several positive coliform test reading just prior to the outbreak, however the system was in compliance with water standards. The system likely became contaminated with surface water. E. coli O157:H7 was never isolated from the water, however water samples from the storage tank collected on July 14 yielded a high coliform count. Entercoccus faecium was isolated from the same water sample, indicating fecal contamination.