Although the determinants of sex differences in territoriality have been addressed for many species, the consequences of such differences in space use, particularly in southern hemisphere taxa, have received limited attention. The Namaqua rock mouse, Aethomys namaquensis, is a medium-sized, omnivorous, nocturnal murid with a wide distribution throughout southern Africa. Sex differences in space use behaviour were found: female captures were described by non-overlapping contiguous areas, whereas males were caught over 50% larger areas that overlapped spatially and temporally. Movements during breeding were characteristic of scramble-competition polygyny: a transient influx of non-resident males (coinciding with an increase in perforate females) skews the sex ratio towards males early in the breeding season. Whereas female condition improved with time, male body condition declined significantly over the breeding period (r²₆₂ = 0.134, P = 0.002) and males had lower persistence in the trappable population. This study may therefore document a link between mobility (associated with a polygynous mating system) and reduced male survivorship and body condition for this murine rodent. Keywords: Body condition index, scramble-competition polygyny, seasonal breeding, space utilization, territoriality.