1. The behaviour of dispersing and resident deer mice (Peromyscus maniculatus) was compared in three laboratory tests to determine if dispersers differed behaviourally from residents. 2. The hypothesis that behavioural differences have a genetic basis was examined by correlating genotype at three electrophoretically detectable blood protein loci with scores on the behaviour tests. Among resident males, level of aggression (as measured in neutral arena encounters) was found to be positively related to weight and breeding condition, and was correlated with genotype. Dispersing males were less aggressive than resident males. Breeding residents of both sexes more readily explored an unfamiliar maze than did non-breeding residents or all dispersers. General level of locomotory activity was not related to breeding condition or weight, and dispersers tended to be more active than residents. 3. It is suggested that two general types of dispersal are common in wild populations of deer mice: subordinate males disperse in response to social pressure during the breeding season, and more active animals of both sexes disperse independently of social pressure, particularly during the nonbreeding season. While genotype probably influences dispersal tendency, particularly tendency to disperse in response to social pressure, without estimates of the heritability of dispersal in wild populations, we cannot assess the relative importance of the genotypic and phenotypic influences.