During the summer of 1975, 285 white suckers (Catostomus commersoni) from 10 localities in southern Ontario were examined for trypanosomes. Trypanosoma catostomi Daly and DeGiusti, 1971 was found in the blood of 11.6% of the fish examined using the haematocrit centrifugation technique. Infected juvenile fish (33%) and infected adult fish (4%) were found in 6 of 10 locations.