The effects of hydrazine on bluegills, Lepomis macrochirus, were assessed in terms of acute toxicity and by examining the alterations hydrazine induced in the dorsal light response and aggressiveness. The static 96-hour median lethal concentration (LC50) of hydrazine was 1.08 mg/liter and the 96-hour continuous-flow no-lethal-effect concentration was 0.43 mg/liter. The dorsal light response in the presence of an artificial prey was significantly decreased within 15 minutes of exposure to hydrazine concentrations well below the 96-hour static LC50. This was true both in static and in continuous-flow conditions. In addition, aggressiveness, as measured by the number of attacks on the prey, was increased in a dose-related manner. Control fish made no attacks, but attacks increased as the hydrazine concentrations increased. These behavioral measurements offer some promise as monitors of subtle, sublethal effects of hydrazine and possibly other aquatic pollutants on fish.