We present results of a radiotracking study of the black-tailed tree rat Thallomys nigricauda, based on 3 males and 4 females in the breeding season and 2 males and 5 females in the nonbreeding season. The study was conducted in the southern Kalahari thornveld, South Africa, a savanna landscape of acacia trees and patchy acacia bush. Nocturnal activity patterns, vegetation use, space use, and interactions were followed for 8–17 days. During the breeding season, males were active with 2 synchronous activity peaks shortly after sunset and late at night. Home ranges included the nests of 8–10 females, 6 different daytime resting places, and 3–5 areas of high activity. Males were mobile within home ranges of 5–10 ha (90% minimal convex polygons) that overlapped with other males and covered home ranges of several females. Activity of females was high shortly after sunset and before sunrise with 6 h of low activity between. Home ranges of nursing females were 0.001–0.03 ha and included 2 small areas of high activity (nest and foraging site) where no other adult females were observed. Animals preferred small acacia shrubs (Acacia mellifera, A. luederitzi) for foraging. During the nonbreeding season, activity of both sexes was low, with only 1 peak shortly after sunset. Although small acacia shrubs did not have foliage, they were preferred by both sexes for foraging, and rats were observed feeding on buds. Home-range size of males was 1–2 ha and of females was 0.1–0.3 ha. Contrary to earlier reports, we found tree rats living solitarily or with young offspring. We suggest that the species has a promiscuous mating system.