On November 12, 2008, the French Institute for Public Health Surveillance was notified by the Centers for Disease Control and Prevention of cases of giardiasis among a group of American tourists returning from a river cruise. An investigation was conducted to identify the source of the infection. Information regarding the tourists’ symptoms and exposures was collected after their return to the United States through standardized interviews. Data on the cruise itinerary and the crew were obtained from the organizing company. Parasite testing was performed on stool samples from tourists and crew members. Environmental investigations of the ships were conducted, and a restaurant frequented by the tourists was inspected. Nineteen cases of giardiasis were identified among the 27 passengers and crew members (attack rate: 70%). Ten tourists and two crew members tested positive for Giardia sp. The investigation results point to swimming in the hot tub on one of the ships or person-to-person transmission from passengers carrying the parasite. This type of cluster of cases is rarely identified in France. The source of the contamination could not be determined due to the delay in reporting this incident and the difficulty in conducting investigations on the ships close to the time of exposure. The hypothesis that the contamination originated from individuals carrying Giardia sp. is all the more plausible given that the first five cases were from Wisconsin, one of the three states most affected by giardiasis in the United States. (R.A.)