On August 23, 2000, the Departmental Directorate of Health and Social Affairs (DDASS) of Lot (46) was alerted by a general practitioner to an unusually high number of cases of gastroenteritis at a vacation center in the town of Gourdon. An investigation quickly ruled out a foodborne cause. Private practitioners in the town contacted by the DDASS also reported a high number of consultations for gastroenteritis. On August 24, 2000, the results of the analysis of water from the municipal water supply, sampled at the vacation center, revealed microbiological contamination. The Lot Prefecture then recommended that residents of Gourdon not drink tap water, or drink it only after boiling it (for at least five minutes). An investigation was conducted by the DDASS in collaboration with the Inter-Regional Intervention Epidemiology Unit (CIRE) of the Southwest and the Institute for Public Health Surveillance. The total number of medical visits and the number of visits for gastroenteritis during the month of August were collected from private practitioners and the emergency department of the local hospital to confirm the existence of an outbreak. A retrospective cohort study of the general population was conducted in the municipality to determine the role of tap water as a source of contamination. An environmental investigation and microbiological tests of stool samples and water from the water supply system were performed. The results of this study confirmed, on the one hand, that this was an outbreak of gastroenteritis caused by multiple pathogens and, on the other hand, revealed a strong association between tap water consumption and the occurrence of gastroenteritis among residents of Gourdon. The environmental investigation revealed malfunctions in the water supply system’s chlorination systems. The identification of human viruses in stool samples and in the water supply suggests a source of fecal contamination. (R.A.)