M.A. / Two species of multimammate mouse, Mastomys coucha and M. natalensis are common, and widely distributed in southern Africa, occurring sympatrically in some areas, and allopatrically in others. The limits of their distribution are only provisional so far. As they share a high degree of morphological similarity, they are, as yet, impossible to identify with certainty in the field. Each species of multimammate mouse carries important diseases: with M. coucha being a carrier for the bacterium causing plague, and M. natalensis carrying the virus causing Lassa fever. In many areas, multimammate mice, being highly adaptable and ecological generalists, have become co-habitants with humans. This fact, coupled to the medical significance of both species, lends importance to being able to identify each species where it occurs, especially in areas where they occur sympatrically. Thus, a total of 40 specimens of M. natalensis were trapped from Richards Bay and La Lucia ridge in KwaZulu-Natal, and 43 specimens of M. coucha from Montgomery Park in Johannesburg and from the shores of the Vaal Dam in the Free State with the aim of comparing these two species via gel electrophoresis. These specimens were from allopatric populations from the centres of their provisional distributions. It was expected that there would be genetic differences between the two sibling species. Blood, liver, and muscle samples were taken, either in the field from dead specimens caught in snap-traps, or back in the laboratory from live-trapped specimens. Fifteen proteins or enzymes provided interpretable results at a total of 39 loci. Nineteen of these were polymorphic