Three hundred and forty seven rats belonging to the Mastomys natalensis complex (Smith, 1834) were live trapped from 60 localities in southern Africa. They were identified as species A, or B using either or both diploid chromosome number and haemoglobin electromorphs. These, and previously published data are presented to give a distribution of the species in southern Africa. It is suggested that workers may safely use the haemoglobin marker for identification purposes in this region as opposed to the more difficult chromosome marker. We suggest that species A can be referred to M. natalensis (Smith, 1834), and species B to M. coucha (Smith, 1836). We consider the two other oldest synonyms, M. caffer (Smith, 1834), and M. marikquensis (Smith, 1836).