The isozymes described in this study provide the first account of the amount, patternand distribution of genetic variation within this genus. Twenty-five enzyme coding loci in two populations of H. vittafus, from Namibia and South Africa, were analysed by horizontal starch gel-electrophoresis. Electrophoretic analysis of liver, white muscle, heart and testis samples revealed genetic variation at 20% (Upper Zambezi River, Namibia) and 36% (Olifants River, South Africa) of the protein coding loci studied. Average heterozygosity values ranged from 1.9% (Upper Zambezi River) to 4.6% (Olifants River), with a genetic distance value of 0.005 between these populations. The low amount of genetic variability in the former population compared to that of other fish species from the same geographical area, and to that of H. vittatus from the Olifants River, can be attributed to restricted gene flow owing to isolation. Although morphological differences exist between the two populations, the extent thereof is not sufficient to regard them as subspecies.