Five clustered cases of leptospirosis were diagnosed in the area of Rochefort, France, in June 2001, among teenagers who had swum in the Genouille canal. The symptoms included fever, headache, abdominal pain and vomiting, chills and myalgia. Three cases were confirmed by PCR and serology. The mean cumulative duration of bathing was significantly higher in cases (23.8 hours) compared to controls (14.4 hours). No other particular risk factor was observed. The environmental investigation revealed the presence of rodents excreting of leptospires near the bathing area. For all antigens considered, the occurence of seropositive rodents was 30.8%, L. icterohaemorrhagiae being the predominant serogroup (23,1%). (R.A.)