During a 22-month exposure of bluegills to copper in soft water, adult bluegill survival was reduced, growth was retarded, and spawning was inhibited at 162 μg/liter of copper (Cu). Tissue-residue analysis of the gill and liver indicated that significant quantities of copper had accumulated in these tissues at test-water concentrations of 40-162 μg Cu/liter. Larval bluegill survival after 90 days' exposure to copper was adversely affected at 40-162 μg Cu/liter. The maximum acceptable toxicant concentration (MATC) for bluegills exposed to copper in water with a hardness of 45 mg/liter (as CaCO3and a pH range of 7-8 lies between 21 and 40 μg Cu/liter. The mean 96-hour TL50 for juvenile bluegills is 1,100 μg Cu/liter, and the application factor (MATC/96-h TL50) lies between 0.02 and 0.04.