A cluster of gastrointestinal illnesses was noted among adolescent residents of a boys' residential treatment facility in Park County, Wyoming. Epidemiologic data suggested that contact with cattle at the facility was the most likely source of the bacterial infection. The facility reported that some calves had diarrhea. At least one staff member reported that residents had brought an ill newborn calf into their living quarters to nurse the animal. Investigators believed that environmental contamination and direct contact with sick calves were the most likely source of the illnesses. Other potential routes of transmission could not be ruled out(i.e. residents cooked meals for one another.) There was no evidence that the facility's well water systems were contaminated.