We measured survival of caged overwintering young-of-the-year and introduced, free-swimming adult smallmouth bass Micropterus dolomieui in five Ontario lakes that varied in pH from 4.9 to 5.9 (winter values). No hatchery-reared young survived in lakes that ranged in pH from 4.9 to 5.9 and in total aluminum from 55 to 215 μg/L. The only lake in which young survived the winter (2–100% survival) had a pH of 5.9 and a total Al of 34 μg/L. Survival of overwintering young in the laboratory was high (92–100%) at all pH levels tested (4.9–6.0). Differences in metal (AI, Cu, Zn, Ni, Mn) concentrations may account for survival differences between the field and laboratory and between lakes with similar pH. Adult smallmouth bass were transferred to four of the same lakes (19–24 fish/lake) used for the experiment on young fish. Population size estimates for the introduced adults were obtained from snorkeling surveys conducted during three spawning seasons (1987–1989). Estimated population size, expressed as a percentage of the number stocked, was highest (41–55%) in the lake with pH 5.9, low (4–12%) in the lake with pH 5.4, and zero in two lakes with pH 4.9–5.2.