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The amelioration of cadmium toxicity by zinc was measured in terms of changes in the maximum specific growth rate (μmax) of a Chlorella pyrenoidosa population using turbidostat culture. In this continuous culture system, μmax is a dependent variable that responds directly to toxic effects. At a sublethal cadmium concentration (2.05 μM Cd) in the medium, the population μmax was in steady state at 70% of control values. When zinc was added to the medium to a concentration of 4.08 μM Zn, μmax increased immediately to a rate of 2.2% of control μmax/generation for seven generations, reaching steady state at 90% of the control. Additional increases in zinc did not elicit further increases in μmax. Full recovery of μmax subsequently occurred after five generations in control medium. Mean cell size and percentage dry mass also changed during periods of toxicity and amelioration. Mechanisms of the amelioration process are discussed.