In December, 1976, an outbreak of gastroenteritis occurred at a resort camp in Colorado. Data obtained by questionnaire from 760 persons indicated that 418 (55%) had had gastroenteritis at the camp or within a week of leaving it, with peak onset within a two-day period. Symptoms included vomiting (81%), diarrhoea (65%), and fever (49%); median duration of illness was twenty-four hours. The attack-rate increased with consumption of water or ice-containing beverages. The camp water supply was found to be inadequately chlorinated and contaminated by a leaking septic tank. Although routine laboratory tests did not reveal bacterial, viral, or parasitic pathogens, immune electron microscopy detected virus-like particles in two of five diarrhoeal stool filtrates. Oral administration of one of these bacteria-free filtrates to two volunteers induced a gastrointestinal illness similar to that observed in the camp visitors.