Abstract. Because rodents behave cryptically and often have large home ranges, the role of social defence in determining their spatial dispersion in grassland remains an enigma. Individual dispersion and access to resources could be determined mainly by the aggressive exclusion of intruders from large territories by residents, or by the scattered distribution of resources and avoidance of dominant competitors occupying preferred sites. The ways in which predictions from these two hypotheses correspond to intra-sexual competitive behaviour within unfamiliar dyads of the mouse Mus spretus, recently captured from two grassland populations, were examined. A series of tests in enclosures examined (1) exploration by intruders given a choice between a resident's soiled sites versus clean sites, or between resident-soiled sites versus sites bearing their own odour, in the absence of the resident; (2) the response of an intruder on meeting either the resident or another mouse, in an adjacent clean tunnel; and (3) the response of a resident to an intruder compared with mice meeting in a clean enclosure. In both sexes, dyads quickly established dominance relationships through brief attacks and chases, and static defensive postures, rather than persistent pursuit and flight. Intruders were strongly attracted to a resident's nest and subsequently were more aggressive towards the resident than towards an opponent from an unfamiliar enclosure. There was less differentiation in competitive behaviour and more mutual fighting between residents and intruders than between mice in clean enclosures, with relative body weight being the most important factor determining competitive behaviour. The results were thus more consistent with competition for dominance over suitable sites than investment in fierce aggression to drive competitors away.