Brants' whistling rat Parotomys brantsii is a gregarious rodent that undergoes marked population fluctuations every few years. Previous studies have variously described them as solitary or social. In this field study, based on direct observations, the social structure and breeding system of a population that almost quadrupled over 7 months was investigated. At the onset of the breeding season, the animals were solitary, each occupying a separate warren. By the end of the breeding season, there was multiple occupancy of some warrens. Each individual maintained a discrete territory within the warren, and exhibited no apparent social behaviour. Outside the breeding season the behaviour of males and females did not differ and both remained at their resident warrens. During the breeding season male P. brantsii visited more neighbouring warrens than did the females, and their visits were to unoccupied warrens or to those occupied by females. Females tended to remain within their own warren. Both sexes avoided warrens occupied by males and few male–male interactions were observed. Females were solicited at their warrens by males, who were frequently chased away. There was, however, evidence that reproductively receptive females were more tolerant of males, who may guard females following mating. This breeding system, based on competitive mate searching by males, and not overt aggression between males, parallels the scramble competition polygyny described for a number of other species, including North American ground squirrels.