Describes an extraordinary endemic radiation of hydrobioid snails in the Mekong River (MR). The monophyletic radiation involving the subfamily Triculinae, 3 tribes, 11 genera, & 92 species, is larger than contemporaneous endemic freshwater gastropod faunas of centers of endemism, such as lakes Tanganyika & Baikal. The study addresses the issue of how higher taxa arise by analyzing the series of adaptive radiations within the MR Triculinae. The worldwide family Hydrobiidae is polyphyletic & includes 2 distinct lineages, the Hydrobiidae sensu stricto & the Pomatiopsidae with the subfamilies Pomatiopsidae & Triculinae. The differences between the two families were masked by convergence in shell, radula, penis, & operculum. Maps & illus.