The family Nemasomatidae is redefined to include onty genera with all sterna secondarily free from pleurotergites. Comments are given on the included genera, viz., Antrokoreana, Basoncopus gen. n. (type-species B. filiformis sp. n.) (Kazakhstan), Dasynemasoma, Thalassisobates, Sinostemmiulus, Nemasoma, and Orinisobates. Isobates coiffaiti Demange, 1961 is synonymized with Thalassisobates littoralis (Silvestri, 1903). Orinisobates is revised and shown to include O. soror sp. n. (Kuril Islands), O. microthylax sp. n. (Kamchatka and Siberia), O. gracilis (Verhoeff, 1933) (NW China), O. sibiricus (Gulicka, 1963) (Altai region, Kazakhstan), O. kasakstanus (Lohmander, 1933) (Kazahkstan), O. nigrior (Chamberlin, 1943) (eastern United States), O. utus (Chamberlin, 1912) (northwestern United States), and O. expressus (Chamberlin, 1941) (northwestern United States and adjacent Canada). Mimolene oregona Chambertin, 1941 and M. sectile Loomis & Schmitt, 1971 are synonymized with O. expressus. A possible case of parthenogenesis in O. microthylax is recorded. Evidence is presented for the following sister-group relationships: Antrokoreana + (Basoncopus + (Dasynemasoma + (Thalassisobates + (Sinostemmiulus + (Orinisobates + Nemasoma))))). The position of Basoncopus is uncertain, and O. soror may belong in a separate genus and constitute the sister-group of Orinisohates + Nemasoma. If soror does belong in Orinisobates, it is the sister-group of all its congeners. The American species of Orinisobates are shown probably to constitute a monophyietic group. The family is suggested to have originated in the eastern Palearctic region, Orinisobates having invaded North America via the Bering Bridge. Doubtful species and species erroneously assoiciated with the Nemasomatidae are listed. The genera Okeanobates and Yosidaiulus are excluded from the family and referred to Okeanobatidae stat. n. in superfamily Blaniuloidea. The genera Trichonemasoma, Telsonemasoma, and Chelojulus are also excluded from the Nemasomatidae and relegated to Julida incertae sedis.