An outbreak of gastroenteritis occurred among visitors and residents of South Bass Island, Ohio. An environmental assessment and extensive testing of island water sources was performed, including use of a novel ultrafiltration water collection system developed at CDC. Of the 1,450 persons reporting illness, Campylobacter jejuni, norovirus, Giardia intestinalis, and Salmonella Typhimurium were laboratory confirmed in 16, 9, 3, and 1 persons, respectively. Cases were more likely than controls to have drunk water on the island The water supply was from a municipal surface water system and ground water wells. Sampling of ground water wells indicated contamination with multiple fecal microbes, including Escherichia coli, Campylobacter jejuni, Salmonella spp, and Giardia spp. Sewage disposal practices likely contaminated the underground aquifer.