Dugesia polychroa (Schmidt 1862)*, a common European triclad, is a recent immigrant to the Great Lakes system (Ball 1969; Mettrick, Boddington & Gelder 1970). Over the past twenty years Reynoldson and his co-workers have added considerably to our knowledge of the distribution, abundance and population biology of the major British triclads, including D. polychroa. Reynoldson (1966a) put forward the hypothesis that the limited distribution and abundance of the triclad species were due to inter-specific competition for food. Similarly, due to intra-specific competition for food, an ability to shrink in size and an absence of predators, the population of any specific triclad showed a restricted fluctuation in population numbers. Consequently British triclad species appear to be living under conditions of food shortage for long periods of the year. Inter-specific competition for food against the Polycelis species has forced Dugesia polychroa to seek a good 'refuge' in the gastropod molluscs (Reynoldson 1966a; Reynoldson & Davies 1970). The distribution and population abundance of D. polychroa in Britain is, therefore, limited by the distribution and abundance of the gastropod food 'refuge'. Ball (1969) suggested that in North America D. polychroa would be similarly restricted by the availability of gastropod molluscs. However, preliminary observations on the fauna of Toronto Harbour suggested that, in the absence of the competing Polycelis species, the abundance of Dugesia polychroa was related to the abundance of the tubificid oligochaetes (Mettrick et al. 1970). The presence of a large population of D. polychroa in Lake Ontario represented an ideal and unique opportunity to study the adaptations made by this immigrant species to the very different ecological conditions of North America. Specifically, the present paper covers aspects of food sources, abundance, distribution and structural changes in the population of D. polychroa in Toronto Harbour over a 20-month period. The results have been compared. with Reynoldson's (1961b) study on this species in Britain.