The Richelieu River, Quebec, is a highly-regulated waterway subject to numerous anthropogenic influences from municipal effluents and agricultural activities. Parasite communities in 234 spottail shiners (Notropis hudsonius) were examined from 4 localities in late spring 2003 and 2004. Component species richness varied between 15 and 18 species in 2003 but declined to 9 to 13 in 2004, while total parasite abundance was consistently lower in 2003. Parasite component community similarity among localities could not be directly linked to available upstream water quality measurements or anthropogenic activity and was best explained by precipitation. Total precipitation in May 2003 was approximately 40% more than in May 2004, presumably altering patterns of runoff, river flow rates, and water quality. This study suggests that fish parasite species composition and richness in the Richelieu River are influenced by environmental parameters which in turn ultimately are driven by a combination of climatic conditions and anthropogenic activities in the watershed.