Three experiments examined the mouse-fighting and cricket-attacking responses of ICR mice reared under various conditions. Groups of mice were raised in isolation, with other mice, or exposed to protected or unprotected crickets. Two aggression testing periods were held when the mice were between 40-45 and 60-65 days of age. Daily aggression tests, consisting of lO-min presentations of a cricket or mouse, were conducted during these periods. Attack latencies were measured on each trial. Analyses of attacks indicated that (1) early experience had little effect on cricket attacks-most mice readily attacked crickets; (2) mice attacked both crickets and other mice faster at 60-65 days of age than at 40-45 days of age; and (3) groupreared mice and female mice were less aggressive towards other mice than were mice raised in isolation or mice raised with crickets.