Because of the cryptic nature of small mammals inhabiting vegetation with widely scattered food resources, little is known about the behavioural mechanisms underlying their spatial dispersion. While house mice exploiting human-built environments attempt to exclude intruders aggressively from small territories that contain concentrated food resources, comparative tests show that the grassland mouse, M. spretus, establishes dominance relationships and then avoids sites used by dominant competitors, probably to avoid displacement from sites that provide safety from predators. We carried out further comparative tests to examine competitive behaviour and response to odours from same-sex conspecifics among house mice living ferally on the Isle of May, U.K., using recently captured adult males (N=62) and females (N=23). In contrast both to house mice caught from buildings and to the grassland M. spretus, feral house mice from the Isle of May showed no aggressive, defensive or cautious behaviour on meeting a same-sex conspecific, with the exception of one aggressive male. Scent marks had no effect on their response to a competitor, although intruders in a scent-marked arena showed a significant sex difference in choice between a clean nest site versus one soiled by a same-sex resident: males spent more time in the soiled nest while females spent more time in the clean one. We propose that the lack of aggression among feral May mice may be due to the infeasibility of defending large territories, containing scattered resources, on an island where there is no real predation risk. We suggest that this hypothesis may explain the lower aggression and higher densities that are characteristic of populations of small mammals, reptiles and birds living on small islands. Copyright 1998 The Association for the Study of Animal Behaviour.