The tench (Tinca tinca) was introduced illegally into Quebec, Canada, in aquaculture operations in 1986. The fish escaped into the Richelieu River in the early 1990s and has since established a reproducing population. Nine of 10 fish sampled in 2000 were infected with parasites, including Raphidascaris acus (80%), Ergasilus megaceros (60%), larval Valipora campylancristrota (20%) and larval Proteocephalidae (20%). The tench were infected with fewer parasite species than those in its native range, in accordance with the enemy escape hypothesis. The copepod E. megaceros is a new Canadian record, with tench being a new host record. Intensity of E. megaceros was positively related to fish weight. It is suggested that V. campylancristrota, a potentially pathogenic metacestode, may be an introduced species that arrived with the tench, although other possible sources of introduction are discussed. The concomittant introduction of the fish and the parasite V. campylancristota may pose a threat to the copper redhorse (Moxostoma hubbsi), an endangered species found almost exclusively in the Richelieu River.