Parasite community structure can change seasonally with shifts in host habitat and in diet. However, anthropogenic activity may influence the natural changes in transmission dynamics of different parasite species. Effects of seasonal and agricultural activity on the parasite communities of newly metamorphosed northern leopard frogs (Rana pipiens) were investigated in July and September 2001 in 5 wetlands, 3 of which were exposed to pesticide runoff from surrounding agriculture. Nineteen parasite taxa were found. Component community richness was consistently high at the pristine reference wetland, whereas the communities at a managed reference wetland remained depauperate. Infracommunity richness increased throughout the season, but more so in frogs resident in agricultural wetlands. Digeneans using frogs as intermediate hosts dominated the communities, although many species were much lower in abundance in September, suggesting mortality of heavily infected frogs. Mean abundance of Haematoloechus spp. was positively related to that of odonate naiads in the frog diet, which appeared to reflect differential second intermediate host availability between reference and agricultural wetlands. Although virtually absent from wetlands in July just after frog metamorphosis, monoxenous nematodes were more prevalent and abundant at agricultural wetlands as the season progressed. Our results suggest that agricultural activity may further facilitate the transmission of monoxenous nematodes as frogs become more terrestrial.