The sibling multimammate mice, Mastomys natalensis and M. coucha, occur throughout southern Africa, exhibiting limited sympatry. We examine body mass and growth characteristics of both species, from birth to 26 weeks of age. Gompertz growth curves were fitted to data for body mass and lengths of head and body, tail, hind foot, and ear. Males of both species exhibited greater asymptotic values, faster maximum growth rates, later age of maximum growth, and extended growth periods than females. However, neither species established body mass dimorphism before weaning, suggesting that differential maternal investment between sexes is not important in preweaning growth. Furthermore, M. natalensis exhibited significantly faster maximum growth rates to reach a greater predicted asymptotic mass than M. coucha. We discuss these results in relation to mating system and interspecific competition.