The manifestations of predatory aggression in response to placing a locust or a cricket nearby was studied on male mice of 12 inbred lines. Predatory aggressiveness was estimated by latency of the attach reaction and the percentage of aggressive mice in each line. Significant interlinear differences were established both in attach latency and in the number of killers. Significant negative correlation was found between the percentage of attacking mice in each line and the latency of the attack. Predatory aggressiveness of the mice increased with experience of attacking, the attack latency considerably decreasing at repeated testing. The number of mice attacking the insects increased with their repeated presentation in the first test 50% of tested mice attacked the locusts, and 76% in the second. The manifestation of predatory aggressiveness differently depends on the species of the victim, mice of different lines attack differently the locust and the cricket.