The capacity of males to attack a passive standard opponent in a resident-intruder test and the preferences in a dark/light choice situation were measured in 200 male mice from 10 different inbred mouse strains. Large strain differences were found for all variables recorded, i.e., the proportion of attacking males, the time spent in the brightly lit box, and the number of transitions between the lit and the dark boxes. A strong negative correlation was found between the first two variables. This result suggests that males of more attacking strains have a higher level of anxiety but do not differ for their level of activity. An involvement of GABA as mediating factor is suggested.