Male mice differing in their genetically determined disposition for isolation-induced intermale aggression were compared on behaviors related to predatory aggression. An ongoing sequence of selective breeding established high-aggressive (Turku Aggressive: TA) and low-aggressive (Turku Non-Aggressive: TNA) lines from an outbred Swiss albino foundation stock. The parental strain, designated the Normal (N) strain, has been kept as a control line and is bred without regard to aggressiveness. Testing consisted of dropping a live cricket into the home cage of the individually housed experimental mice. Results showed that the TA males displayed shorter latencies to attack and spent more time in chasing, attacking, and consuming crickets than did TNA and N males. The TNA males displayed significantly less predatory aggression than both the TA and N males. When brothers of the males tested for predatory aggression were tested for intermale aggression, a similarly significant effect of breeding line was obtained for the latency to attack. Testing consisted of placing an intact male mouse into the cage of the male to be tested. The results suggest that there may be parallels in genetic variation between intermale and predatory attacking.