A report of an increase in cases of acute gastroenteritis (AGE) by the Pérignat-lès-Sarliève medical practice in late June 2011, combined with widespread contamination of the drinking water supply, suggested an outbreak of waterborne AGE. Epidemiological and environmental investigations were conducted to characterize the outbreak, assess patients’ healthcare-seeking behavior, and confirm the waterborne origin. The environmental investigation described the circumstances surrounding the onset of contamination in the water system and identified potential sources of contamination. The epidemiological investigation (n=433 subjects) confirmed the association between the onset of AGE and the consumption of tap water (RR=2; 95% CI [1.3–3.9]) as well as an increase in this risk with the amount of water consumed. The overall attack rate observed was 28%. One-third of cases consulted a doctor, and self-medication was the most frequently observed method of taking medication (40% of cases). Microbiological examination of stool samples was positive for a single pathogen: Campylobacter jejuni. Subsequent microbiological testing of water samples revealed the presence of Shiga toxin-producing E. coli (STEC). This report led to several recommendations regarding the safety of the water supply system and the improvement of detection and investigation methods for waterborne infection outbreaks. (R.A.)