We tested for correlations between the geographic, demographic, and temporal distribution of an aquatic insect host and the prevalence of its gut parasites in southwestern Ohio. Trypanosomatids were present in Aquarius remigis collected from all 4 streams surveyed in the watershed. Prevalence declined dramatically from May to July and remained low through the fall. This pattern was consistent over all sites of our study, with no effect of stream, stream site (upstream vs. downstream), or host sex on prevalence. Stage, however, was strongly correlated with prevalence; adults were more likely to be infected than were nymphs. We argue that behavioral differences between the 2 age classes may account for the decline in prevalence; opportunities for transmission are highest in the spring, when mating activities increase adult host contact rates, and decline in the summer, when contact rates decrease.