Many small mammals inhabiting vegetation habitats where food resources are scattered have large home ranges with limited overlap. One way that they may effectively defend large territories is to establish dominance over the limited number of sites that provide good protection from predators, since displacement from these sites could have a very high cost to intruders. To examine this hypothesis we studied the fine-scale use of habitat and spatial dispersion of all adult male Mus spretus inhabiting a 1.1 ha grassland study site near Lisbon, Portugal, by radio telemetry, at the start of the breeding season. The location of each of the 10 males was mapped every hour, 24 h/day for up to seven days. Microhabitat characteristics were compared between a random sample of points in the study site and those where mice were found. Individual ranges did not overlap, despite the close proximity of their borders and the occupation of almost all suitable habitat, suggesting that individual dispersion was strongly influenced by the presence of neighbours; mean range size was 343 ± 95 m2. Residents covered less than one-third of their total range over 24 h, though neighbours did not intrude despite the apparent opportunities. Each male territory overlapped the territory of at least two females. Mice were neither nocturnal nor crepuscular, moving around mostly during the morning and evening. They avoided open woodland or pathways, preferring grassland sites with tall vegetation and sites where shrubs, bramble, or dead wood provided additional cover. Most fixes per male (70%) were located in one to four core areas, which represented only a tiny proportion of each range (6.9 ± 0.9%). Although exclusive defence of large complex ranges is likely to be impracticable, defence of core areas seems much more feasible. Our results thus support our hypothesis that mice may be able to maintain large exclusive ranges due to a combination of high predation pressure and a limited number of sites with sufficient ground and overhead cover. This will result in a very high risk to mice entering areas where competitors have priority of access to protected sites.