On March 31, 2012, the emergency medical services reported an outbreak of gastroenteritis among students at a middle school who were returning by bus from a ski trip in the Hautes-Pyrénées from March 26 to 30, 2012. The Regional Health Agency (Cire) for the Limousin and Poitou-Charentes regions conducted a thorough investigation to identify the type of infection involved, the source and possible vehicle of transmission, as well as the extent of the outbreak. A retrospective cohort study was conducted among the middle school students, the supervising staff, and the bus drivers for the ski trip. Self-administered individual questionnaires were used to collect information on clinical symptoms and foods consumed. The center director and the catering provider were contacted as part of an environmental investigation. The response rate to the survey among students was 93 out of 96 (96.8%). The overall incidence rate was 46 out of 96 (49.5%). Four cases were hospitalized, and there were no deaths. The epidemic curve spanned 8 days, peaking at 20 cases (47.6%) on the night of the return bus trip. The first case became ill on the first day of the ski trip and is potentially the source of the outbreak. Vomiting (44 cases, 95.7%) was reported more frequently than diarrhea (13 cases, 28.3%). The median duration of illness among cases was 34 hours (IQR: 12–55 hours). No stool cultures could be performed on the hospitalized individuals. The results show a clustering of cases by room based on the date of symptom onset. Questions regarding the consumption of specific foods analyzed for Wednesday, the 28th, and Thursday, the 29th, yielded no results. Despite the lack of laboratory confirmation, the results allow us to conclude that the reported public health event was an outbreak of viral gastroenteritis. A foodborne or waterborne origin is unlikely. The primary mode of transmission was likely direct contact with an infected person. The presence of the virus in vomit, accompanied by the formation of aerosols, likely also played a role in the rapid spread of the virus. Indirect contamination through contact with contaminated surfaces, as well as with the necks of individual contaminated bottles reused by students, is also likely. (R.A.)