From March to June 2008, nine cases of acute hepatitis A, all occurring in the same municipality within the Lille metropolitan area, were reported to the Departmental Directorate of Health and Social Affairs (DDASS) of the Nord department: five cases in school and family settings, and four cases within a traveling community in the municipality. An epidemiological investigation was conducted to identify factors contributing to transmission and to adapt control measures. Epidemiological data were collected from mandatory reports and through telephone interviews. An environmental investigation was carried out at the affected reception area. A confirmed case was defined as a person attending school or residing in the municipality who presented with acute hepatitis A with positive anti-HAV IgM antibodies between March 2008 and February 2009. A total of 54 cases were identified between March 2008 and February 2009. The median age of the cases was 8 years [min.-max.: 2–42] and the male-to-female ratio was 1.2. No severe cases were reported. The most likely source of infection was the home (54%), within the family (26%), and within a community setting (7%). Over the course of the outbreak, the same genotype IA strain was identified in 3 cases. In addition to the usual recommended preventive measures, a vaccination campaign was implemented among the Traveller community. The shape of the epidemic curve and the epidemiological links between cases suggest person-to-person transmission, which is particularly active among populations living in poor sanitary conditions. This episode illustrates the value and necessity of early implementation of mass vaccination of at-risk populations living in poor sanitary conditions as a control and prevention measure complementary to the reinforcement of individual and collective hygiene measures. (R.A.)