As a result of examination of the fund collection of black flies (Diptera: Simuliidae) of the Zoological Institute of the Russian Academy of Sciences, detailed redescriptions and original drawings of 9 species (of 12 known in the World fauna) of the genus Gymnopais Stone, 1949 and 4 species (of 10 known in the World fauna) of the genus Twinnia Stone et Jamnback, 1955 are given. Keys to all known12 species of Gymnopais and 9 species of Twinnia of the World fauna are provided. The information on Twinia changbaiensis Sun, 1994 (China, Liaoning) is insufficient, and this species is not included in the key.