Underutilized ecological niches provide ecological opportunity and thereby promote diversification. Postglacial lakes are known to provide such favourable conditions for evolutionary diversification and adaptive radiations of fishes. Several postglacial radiations of Arctic charr Salvelinus alpinus (L.), whitefish Coregonus sp. and stickleback Gasterosteus aculeatus (L.) are described in the literature. The Arctic charr is known to frequently exhibit two to four sympatric morphotypes. In this study we investigated the patterns of diversity of Arctic charr within a drainage system on the Vatnahverfi peninsula in Southern Greenland. We found exceptionally high morphological diversity, with up to five sympatrically occurring morphotypes. In the largest lakes, a planktivore, a littoral benthic, a profundal benthic, an anadromous and a piscivorous type could be distinguished. These morphotypes did not only differ in their morphology, but were also ecologically and genetically distinct. Moreover, one morphotype was subdivided into two sympatric, genetically clearly distinct clusters that also tended to differ in morphology, and partly in diet. Considering the clear distinctiveness of the ecomorphs in the large lakes I suggest regarding them as separate species. Different morphotypes could also be distinguished in one of the small and isolated lakes but, in contrast to the large lakes, the morphotypes were not genetically distinct and indicated rather individual specialization and ontogenetic niche shifts other than ecological divergence. Consistent with predictions of island biogeography theory, the highest charr diversity was observed in the largest and deepest and presumably also least isolated lake at low altitude.