Home-range size may reflect the physiological state and energetic requirements of an individual. In the black-tailed tree rat (Thallomys nigricauda) we expected differences in the home-range size of populations in habitats of differing productivity, as measured along an aridity gradient, and between seasons. We studied the variations in home-range size of black-tailed tree rats by radiotracking in winter 2006, 2007 and summer 2007 at three study sites along an aridity gradient in southern Africa. Home ranges were estimated using 100% and 95% minimum convex polygons (MCP) and 95% and 50% fixed kernels. Home ranges varied widely, from 166 to 80 199 m² for males and from 46 to 8810 m² for females. Males had larger home ranges than females, which supports a promiscuous mating system reported for the species. Although range size was reduced in both sexes in winter, this was not significant. We found no significant difference in home-range size along the aridity gradient. It is suggested that a combination of habitat productivity, season and breeding system influences the size of home range and that this species displays phenotypic flexibility in terms of its behavioural responses to these factors.