On July 28, 2009, a cluster of influenza cases at a vacation center in the Alpes-de-Haute-Provence region was reported to the InVS regional office in the South. An epidemiological and biological investigation was conducted to identify the source of the infection and estimate the prevalence of the new virus. The center housed 94 children divided into five age groups and 28 adults. Cases were identified based on the clinical definition of a possible influenza case established by the InVS. Nasal swabs were collected from all individuals present and tested for the 2009 A(H1N1) virus. The overall attack rate was 37% for probable cases and 21% for confirmed cases. All probable cases had presented with a cough, and 82% had at least one episode of fever. Attack rates varied by age group, and the risk of developing the disease was thus significantly higher among 11- to 14-year-olds than among 6- to 8-year-olds. Of the 111 samples tested, 31 were positive, including 5 from "non-cases." This study examined the dynamics of the epidemic’s spread within the vacation center and the influence of age-group-specific behaviors on its transmission. It also confirms the presence of viral carriage among pauci- and asymptomatic individuals. (R.A.)