The taxonomy of 8 of the 1 1 genera of the beetle family Hydraenidae in southern Africa is presented. This fauna, previously virtually unknown to science, includes both aquatic and humicolous forms, the latter morphologically and ecologically very unusual in the family. The fragile habitats of endemic forms, especially those of the southwestern Cape, are discussed relative to the biodiversity crisis. A highly endemic and highly morphodiverse lineage, the Prosthetopinae (with related species in Madagascar) is indicative of an afromontane biogeographical region comprised (in part) of Gondwanaland relicts. One subfamily, 5 tribes, 5 genera, and 38 species are described as new. Taxonomically and phylogenetically significant structures are illustrated with line drawings or scanning electron micrographs, and geographical distributions are mapped. Natural history data are noted for genera and, when available, species. Diagnostic keys to tribes, genera, species groups, and species are given. Morphological features unique to the subfamily Prosthetopinae are discussed and illustrated.