Ph.D. / Since the systematics in the family Characidae from southern Africa were only reliant on some morphological traits, the determination of the genetic variation, genetic distances and phylogenetic relationships using different molecular techniques led to the formulation of this study. The first aim of the study was to describe the amount and pattern of genetic variation withi~ and between populations. The analysis of the isozymes provided the first account of the amount. pattern and distribution of genetic variation within this family. A general introduction on the history, distribution, descriptions, taxonomy and some ecological notes from the literature on southern African Characidae species is included in Chapter l. The second chapter dealt with the Hydrocynus genus that represents, morphologically, the largest member of this family. Other members of the Characidae family are relatively small species that do not exceed 300g. Twenty-five enzyme coding loci in two populations of H. vittatus, from Namibia and South Africa, were analyzed by horizontal starch gelelectrophoresis. Electrophoretic analysis of liver. white muscle, heart and testis samples revealed genetic variation at 20% (Upper Zambezi River. Namibia) and 36% (Oiifants River. South Africa) of the protein coding loci studied. Average heterozygosity values ranged from l .9% (Upper Zambezi River) to 4.6% (Oiifants 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 due to isolation. Although morphological differences exist between the two populations. the extent thereof is not sufficient to regard them as subspecies.