Introduction - On November 12, 2008, the French Institute for Public Health Surveillance was notified by the U.S. Centers for Disease Control and Prevention (CDC) 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. Materials and Methods - Information regarding the tourists’ symptoms and exposures was collected after their return to the United States. Parasite testing was performed on stool samples from some tourists and crew members. Environmental investigations of the ships were conducted, and a restaurant frequented by the tourists was inspected. Results - From October 2 to 9, 2008, 19 cases of giardiasis were identified among the 27 passengers and crew members (attack rate 70%). The parasite was identified in the stool samples of 10 tourists and two crew members. The results of the investigations pointed to contamination from swimming in the hot tub on one of the ships and human-to-human transmission from passengers carrying the parasite. Discussion-Conclusion - 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 infection originated from individuals carrying Giardia sp. prior to the cruise 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.)