Very little is known about the reproductive biology of the Namaqua rock mouse (Aethomys namaquensis), despite its wide distribution and its being a major component of small mammal communities in southern Africa. Consequently, body mass, reproductive-tract morphometrics, gonadal histology, and plasma progesterone and 17β-estradiol concentrations in females, and plasma testosterone concentration in males were studied over 12 consecutive months in wild-caught A. namaquensis from a population in northeastern South Africa in an attempt to gain insight on the pattern of reproduction in the species. The number of graafian follicles and corpora lutea, and plasma progesterone and 17β-estradiol concentrations in 102 females increased significantly between September and early March relative to between April and August. Gravid and lactating females were observed between October and early March, whereas no lactating females were recorded between April and September. Although testicular mass relative to body mass, testicular volume, seminiferous tubule diameter, and circulating plasma testosterone concentration increased significantly between September and February relative to between March and August, little or no spermatogenesis or presence of sperm in the epididymis were observed between March and September. Examination of these data suggests that the Namaqua rock mouse in northeastern South Africa is a seasonal breeder, with reproduction confined to the rainy summer months of the Southern Hemisphere when nutritional supplies are abundant.