Median lethal concentration (96-h LC50) values for acute copper toxicity to 3-10 g cutthroat trout (Salmo clarki) have been determined for nine different combinations of alkalinity, hardness, and pH. Equilibrium calculations were performed on the copper LC50 values; seven different soluble species of copper were considered: Cu2+, CuOH+, Cu(OH)20, Cu2-(OH)22+, CuHCO3+, CuHCO30, and Cu(C03)22-. The acute toxicity of copper was inversely correlated with water hardness and alkalinity. At a given alkalinity, hardness determined the LC50; at a given hardness, alkalinity determined the LC50. At a given alkalinity and hardness, the concentrations of the copper species were determined by the pH of the water. Under the conditions tested, Cu2+, CuOH+, Cu(OH)20, and Cu2(OH)22+ were toxic forms of copper to cutthroat trout; CuHCO3+, CuCO30 and cu(CO3)22- were not toxic. Results of 11 96-h copper toxicity bioassays on 1- to 26-g rainbow trout (Salmo gairdneri) under uniform water chemistry conditions also reported.