Individual male mice of several groups were observed during daily encounters with a male intruder. The groups differed with regard to social and sexual experience of the resident animal and of the intruder. Aggressive behavior was most intense in residents actually living with a female and least intense in sexually naïve residents living alone. Residents that had only once cohabited with a female for a short time obtained intermediate scores. Experienced intruders were attacked less than naïve ones.