Reproduction and postnatal development of the southern African bush Karoo rat Otomys unisulcatus was studied in the laboratory. Fifty-one litters were obtained from 17 pairs, with a mean litter size of 2.09, reproductive effort of 0.58 and gestation period of 37 days. Neonates weighed 6.1–9.6 g and were semi-precocial. Weaning began as early as 8 days of age, and the age at first reproduction was 6 weeks in males and 5 weeks in females. Compared to its congeners, O. unisulcatus has small litters of semi-precocial young which undergo rapid postnatal development, and contrary to expectations displays several K-selected reproductive characteristics. The reproductive biology and postnatal development of the bush Karoo appears to reflect a compromise between exploiting sporadically abundant resources and the occupation of stick lodges, which may provide a favourable microclimate to buffer the harsh external climate.