Alert. On August 11, 2008, the Ardèche Departmental Directorate of Health and Social Affairs (DDASS) was alerted to the occurrence of numerous cases of acute gastroenteritis (AGE) at a campground in Vallon-Pont-d'Arc. An investigation was conducted to confirm the existence of an outbreak, identify its cause, and implement appropriate control measures to stop the outbreak. Method. A retrospective cohort study was conducted to collect information from campground guests regarding clinical symptoms and potential risk factors. The DDASS carried out an environmental investigation. Results. Of the 276 people surveyed, 142 cases of AGE were identified (an attack rate of 51.5%). The risk of developing GEA was higher among those who consumed water from the campground and increased with the amount consumed. Despite the campground’s connection to the public water supply, the environmental investigation revealed that the non-potable water distributed came from an unauthorized private well. Since no stool cultures were performed, the causative agent was not identified. Conclusion. Consumption of non-compliant water was responsible for a major outbreak of AGE at the campground. Preventive measures (prohibition on consuming water from the system) and corrective actions taken as soon as the use of an illicit water source was suspected helped to contain the outbreak. The source of the contamination could not be determined. Several etiological hypotheses can be considered, notably the infiltration of sewage system water into the well; human-to-human transmission may have sustained and spread the outbreak. (R.A.)