The osteology of the gobioid fish Rhyacichthys aspro (Valenciennes, 1837), from tropical hill-streams of the Indo-Australasian archipelago, is described. Adaptive features of external form and skeleton are considered with reference to life in torrential conditions. The systematic position of Rhyacichthys in the suborder Gobioidei is confirmed, and structure of the caudal skeleton in this genus, with other features, is found to support an acanthopterygian rather than paracanthopterygian origin for the gobioid fishes. In lateral-line system (here redescribed) and caudal skeleton, Rhyacichthys is shown to possess primitive features unique among gobioids. Previous systems of internal classification for the Gobioidei, and the inadequate fossil record of the group, are summarized, and the systematic value of certain skeletal criteria discussed. The suborder is now reclassified on a phylogenetic basis into two families, Rhyacichthyidae and Gobiidae, with the latter divided into seven sub-families: Eleotrinae, Pirskeninae, Xenisthminae, Gobionellinae, Tridentigerinae, Gobiinae, and Kraemeriinae.