During the spring, summer, and fall of 2003 and 2004, 947 and 1,970 Planorbella trivolvis (=Helisoma trivolvis) collected from 1 site in Portage County, Wisconsin, U.S.A., were measured and examined for larval trematodes. Cercarial morphology was used to identify trematode larvae, and the prevalence of each larval type was determined. Totals of 292 and 618 larval trematode infections, including 140 and 191 identified as Ribeiroia ondatrae, were observed in 2003 and 2004, respectively. In addition to R. ondatrae, collections yielded 8 morphologically distinct larvae: echinostome, armatae, magnacauda, amphistome, cystophorous, longifurcate-pharyngeate/strigea, brevifurcate-pharyngeate/clinostomoid, and brevifurcate-apharyngeate. Ribeiroia ondatrae and echinostome larvae contributed 30.9% and 25.2%, respectively, of the overall trematode prevalence observed in 2004, whereas magnacauda, cystophorous, longifurcate-pharyngeate, brevifurcate-pharyngeate, and brevifurcate-apharyngeate infections were only infrequently encountered. Because the prevalence of R. ondatrae might have implications for the genesis of amphibian malformations in the field, the focus of this study was to determine how the prevalence of R. ondatrae fluctuates with time. In both 2003 and 2004, the total intramolluscan trematode prevalence, as well as that of R. ondatrae and echinostome larvae, increased significantly from spring to fall. These results were reaffirmed in an analysis of snail size and trematode prevalence, which showed that intramolluscan prevalence increased from spring to fall within all size classes. However, unlike the prevalence of echinostome larvae, the prevalence of R. ondatrae exhibited a midsummer decline before rising rapidly in late September and October.