Abstract The sensitivity of bacterioplankton to copper was analyzed to see whether initial steps in the selection of copper-tolerant life-forms in mixed populations of bacteria were accompanied by changes in basic metabolic parameters. Analysis took place by measuring the incorporation of [3H]thymidine and [3H]leucine, and the hydrolysis of leucyl-β-naphthylamide over a period of 4 d. In acute toxicity tests the radiochemically determined parameters showed the same sensitivities to copper, whereas in the enzyme test the dose-response curve had a much lower slope, indicating less sensitivity. Marked differences were observed in the susceptibility of the different processes after prolonged exposure to copper. Incorporation of [3H]thymidine, [3H]leucine, and proteolytic activity changed substantially during exposure to concentrations as low as 2 to 31 μg Cu L−1. Higher copper concentrations (126-1,000 μg Cu L−1) led in the course of 24 to 48 h to the development of a bacterial community with a higher overall copper tolerance. In winter, these successive events in bacterial populations were observed in the absence of substantial populations of algae or zooplankton. In summer, the metabolic changes in bacterioplankton exposed to copper were strongly affected by the poisoning of other organisms, notably algae, and the subsequent release of organic material. Thus, moderate copper concentrations alter the metabolic profile of bacterial communities, probably as an initial step in the selection of tolerant life-forms.