Xenomystax atrarius Gilbert has been considered a possible representative of as many as three eel families (Congridae, Muraenesocidae, and Nettastomidae) by recent authors. Although the species is described in detail in this account, it cannot be definitely assigned to a family without major revision of these families. The species is now known from 19 specimens captured mostly near the continental slope of the eastern Pacific Ocean between Valparaiso, Chile, and Vancouver Island, B.C. Xenomystax rictus Garman 1899 should be considered a junior subjective synonym of X. atrarius Gilbert 1891.