Approximately 3000 people in Bennington, Vermont, 19% of the population, had a diarrheal illness during the first and second weeks of June 1978. Persons affected reported abdominal cramps (86%), diarrhea (82%), malaise (73%), and headache (47%). The mean duration of illness was 4.6 days. The illness was associated with drinking unboiled water from the town water system. Investigation showed that the entire water system was probably contaminated and the source of contamination was the main unfiltered water source. Bolles Brook. Campylobacter fetus subspecies jejuni was cultured from 15 of 42 rectal swabs obtained from persons who had been ill, but not from swabs obtained from 23 persons who served as controls. No environmental samples of water and no specimens from wild or domestic animals were positive for Campylobacter. This appears to be the first waterborne outbreak of gastroenteritis associated with C. fetus subspecies jejuni.