In 1991, mallards (Anas platyrhynchos) on North and South Lake Leelanau in Leelanau Co., Michigan were trapped, examined for natural infections of avian schistosomes, and treated with praziquantel. Prevalence of infection from the 113 birds recaptured in 1992 was compared with that of the previous year on these 2 inland lakes after some of the birds were treated. Infections were determined by examining diluted fecal samples for hatched schistosome miracidia. Approximately 15% of the 366 birds captured the first year homed the second and made up about 27% of the resident mallard population. Of the birds treated in 1991, only 1.8% showed an infection in 1992 compared with 14.6% for previously untreated birds. These data indicate that praziquantel is an effective therapeutic agent for reducing natural infections of the parasite in mallards. Furthermore, yearly treatment of mallards at specific sites may not be necessary for effective control of swimmer's itch on North and South Lake Leelanau.