In order to identify leech species prevalent in Lake St. Clair, Michigan, U.S.A,. and understand their effect on fish hosts, fish were collected during May 2006 and 2007 from Anchor Bay, Lake St. Clair in the Laurentian Great Lakes Basin. Throughout the study, 2,117 fish from 21 species were examined for the presence of leeches. Overall, 1,064 leeches were collected from 165 individual fish and identified morphologically. Hosts included the channel catfish, freshwater drum, northern pike, northern shorthead redhorse sucker, quillback sucker, rock bass, smallmouth bass, walleye, and yellow perch. Leeches attached to hosts had a prevalence of 7.79%, with a mean intensity of 6.45 leeches/infected fish and an abundance of 0.50 leeches/examined fish. Collected leeches were identified as Actinobdella pediculata (Glossiphonidae), Placobdella montifera (Glossiphonidae), and Myzobdella lugubris (Piscicolidae), which was the most commonly occurring species. The freshwater drum had the highest prevalence, mean intensity, and abundance of leeches as a host for all leech species, as well as for A. pediculata and M. lugubris. Placobdella montifera was rare in occurrence, with little variation in host prevalence, mean intensity, or abundance. This is also the first record of the northern shorthead redhorse sucker as a host for A. pediculata. Leeches were found attached to various sites on the hosts, but occurred primarily on the pectoral fins. Gross inspection showed that leech attachment occurred in high intensities associated with necrotic areas and hemorrhages, and also caused swelling and prevented the opercular flap from closing. Histopathologically, leech attachment caused an extensive inflammatory response, necrosis of the muscle tissue, and edema. Results indicate that there are 3 predominant leech species parasitizing fish hosts in Lake St. Clair, that the leeches have preferred hosts and attachment locations, and that they cause damage to the underlying skin and musculature at the site of attachment.