On July 28, 2009, an outbreak of clustered influenza cases at a vacation center in the Alpes-de-Haute-Provence region was reported to the Southern Regional Office of the French Institute for Public Health Surveillance. An epidemiological and biological study 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 at the time of the study by the French Institute for Public Health Surveillance. Nasal swabs were collected from all individuals present and tested for the A(H1N1)2009 virus. The overall attack rate (AR) was 37%. All possible 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. Among the possible cases, 26% were confirmed. In total, 31 people tested positive for influenza A(H1N1), including 5 who did not meet the case definition (7%). Three of them were paucisymptomatic, and 2 remained completely asymptomatic. 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 asymptomatic and paucisymptomatic carriers of the virus. (R.A.)