Knowledge of home-range sizes and the degree of spatial overlap between males and females can help elucidate mammalian mating systems and social organizations. To characterize the social system and mating strategies of an endangered species, the giant kangaroo rat (Dipodomys ingens), we compared home ranges of males and females during the breeding and nonbreeding seasons using radiotelemetry. Boundaries and sizes of home ranges varied seasonally for males, but not for females. During the nonbreeding season, both males and females remained in exclusive territories located in the core of each individual's home range. During the breeding season, home-range size of males increased significantly as mobility of males increased and home ranges expanded to overlap neighboring territories of females. Home ranges also were more uniformly distributed than in the nonbreeding season, and nearest neighbors were significantly more often opposite-sex individuals. Males likely increased home-range sizes to overlap with multiple females and to enhance their opportunities for mating, perhaps by becoming familiar with neighboring females and monitoring those females for signs of receptivity. Although home ranges of females remained similar in size throughout the year, females seemed able to adjust their home ranges in response to neighboring vacancies. We conclude that spacing of D. ingens is flexible in order to meet changing social and environmental conditions.