The effects of three concentrations of copper (0.034, 0.057, and 1.30 mg/L) on the behavior of social groups of bluegills Lepomis macrochirus were examined. Social hierarchy formation was similar in all test groups. Ten behaviors, including respiratory disruptions and changes in comfort movements and aggression, were monitored. At the group level, there was a significant positive dose-response relationship demonstrated for coughs, yawns, fin flicks, nips and threats. The most subordinate and most dominant individuals of the hierarchy were affected more than the other fish. Subordinate fish had higher cough, yawn, and fin-flick frequencies during treatment than all other fish in the population; the dominant fish had the next highest frequencies. The aggressiveness of the dominant fish in each tank increased after copper exposure. The observation of significant behavioral changes at copper concentrations as low as 0.034 mg/L demonstrated that behavior can be a sensitive indicator of copper stress.