Fish population parameters: abundance, age-class composition, and size-at-age, which respond to environmental stresses, were investigated in two lakes showing chemical and biological evidence of acidification. Plastic Lake, Muskoka District of Ontario, shows spring pH depression and A1 toxicity in surface waters, with an associated annual fish kill. In Crosson Lake, pH 5.2 to 5.3, there has been a dramatic decline in abundance of the white sucker (Catostomus commersoni). In Platic Lake the abundance of pumpkinseed (Lepomis gibbosus) has declined and that of rock bass (Ambloplites rupestris) has increased. In Crosson Lake size-at-age has increased in yellow perch (Perca flavescens), pumpkinseed abundance has increased but with no change in growth rate.