All 12 species of fishes collected from lakes on the Quebec–Labrador geosyncline and the surrounding Canadian Shield exhibited diplostomiasis. Based primarily on data from Salvelinus fontinalis, Catostomus catostomus, and C. commersoni, the infection was rare or absent in lakes with low calcium ion concentrations but apparently ubiquitous in waters high in calcium. The occurrence of this disease reflects the local distribution of the snail intermediate host Lymnaea arctica, which appears restricted to waters with calcium levels of more than about 5 ppm. Such lakes characteristically overlie rock rich in dolomite. This information suggests that the distribution of diplostomiasis in northern Quebec can be predicted from existing geological maps.