Young adult males of the inbred mouse strains BALB/c, C57BL/6, CBA, C3H/He, and the outbred strain NMRI showed significant interstrain differences for isolation-induced aggression as measured in a standard-opponent test. Additionally, four related inbred strains of common origin (ABG, AB//Halle, ABB, AB//Jena) were studied. ABG and AB//Halle showed nearly no overlap for the trait isolation-induced aggression. Tests of histocompatibility and coat color, as well as mandible measurements, confirmed a close genetic relationship between ABG and AB//Halle. The large difference for isolation-induced aggression between these two closely related strains offers excellent opportunities to search for single gene correlates of aggression.