Locomotor activities of mosquitofish (Gambusia affinis) and goldfish (Carassius auratus) were monitored for a 2-week period in the presence of insect growth regulators, at concentrations approximately 10 fold greater than those generally recommended for application. Methoprene, the active ingredient in Altosid SR-10, at 0.2 ppm did not significantly alter the locomotor activity of either mosquitofish or goldfish. Diflubenzuron (trade name, Dimilin) at 0.2 ppm caused a temporary hyperactivity in mosquitofish. Within 2 days following exposure the fish became about 2.5 times more active than controls. Maximum activity was observed on days 4 through 8 when they were 4 times as active as controls. Activity then decreased to control levels by day 14, suggesting that the mosquitofish were able to adjust to or compensate for the presence of diflubenzuron.