Following the notification of nine hepatitis A cases partly clustered near an oyster growing area in the Côtes d’Armor district (Brittany, France), epidemiological, environmental and microbiological investigations were performed in order to identify the source and vehicle of contamination and devise control measures. The investigations identified 111 cases, all of whom lived or had stayed in the district. Of these cases, 87% had eaten raw bivalve molluscs and 81% had specifically eaten oysters. The raw molluscs originated from a single shellfish farm located at the north of Paimpol bay. The shellfish were probably contaminated while being stored in submersible tanks or purified in a land-based tank. They were tainted either by sewage overflows or releases from a polluted storm sewer or effluents from on-site sanitation facilities. To prevent future food-borne outbreaks due to shellfish consumption, hazards should be analysed for each farm of this area. Sewage collecting and treatment facilities should be up-graded around the bay and timely information on sewage overflows should be provided.