Persons who had stayed at, or dined in, a snowmobile lodge in Wyoming developed gastroenteritis. A retrospective cohort study found a significant association between illness and the consumption of well water at the lodge. Norovirus was isolated from eight persons and from a well water sample at the lodge. The outbreak continued over the course of ten weeks. No food was ever implicated with the illnesses. Some foodhandlers reported that they had been ill while working and may have amplified the outbreak. Effluent supplied from an inadequate septic system likely contaminated the well water.