Ancient lakes are among the most stable freshwater environments on our planet, with a species richness far exceeding that of younger and less stable limnic habitats. Compared with the African rift lakes, the ancient lakes of the Indonesian island Sulawesi have received less attention, and their fauna has not been satisfactorily explored. Here, we present results on the freshwater crabs from these lakes and their phylogenetic relationships. Complementing recent descriptions of new species and genera of freshwater crabs from the Parathelphusa complex endemic to the Malili lake system and Lake Poso, here we report on another new species belonging to the genus Sundathelphusa (Potamoidea Gecarcinucidae), so far only known from one locality in Lake Poso. The morphology of the chelae of this crab is indicative of a predatory lifestyle: it probably feeds on the rich gastropod and bivalve fauna of the lake. So far, no specialized molluscivore crab has been known from Lake Poso, whereas mollusc-feeding crabs had been described from all lakes belonging to the nearby Malili system. A phylogenetic reconstruction of all known freshwater crab species from the ancient lakes, based on 562 base pairs of mitochondrial DNA (large ribosomal subunit 16S rRNA), revealed that the crabs inhabiting these lakes are not monophyletic. The large number of endemic crab species in both lake systems is thus not the consequence of a single adaptive radiation, but is the outcome of at least two independent colonizations from nearby tributaries and the subsequent specialization to the stable freshwater environment. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 154, 211–221.