Space use in mammals may vary between the sexes. This may reflect demographic or reproductive differences between the sexes as well as different responses to changes in resource availability. We present the results of a 2-year study on the spatial organization of the yellow-necked mouse (Apodemus flavicollis) in a beech woodland in the eastern Italian Alps. We used radiotelemetry to monitor the movements of 64 mice during the breeding season (i.e., from July to October) in 2005 (high population density) and 2006 (low population density). In both years, home ranges of males were significantly larger than those of females and overlapped with the areas occupied by several individuals of both sexes. Females monopolized core areas and never shared burrows with other females—suggesting intrasexual territoriality—although their home ranges overlapped those of several males. Space use changed seasonally and among years, suggesting a relationship with resource abundance and distribution. Females exhibited reduced spatial exclusivity and larger home ranges during lower food availability; males varied their spatial distribution accordingly by also expanding their home ranges. After a decrease in habitat quality, we observed substantial and abrupt adult dispersal by both sexes. In sum, females varied their spatial and social relationships in response to environmental conditions, whereas males appeared to vary patterns of space use in response to females.