Diasemocera Bezzi, 1895 (Type species: Psilopa nigrotaeniata Bezzi, 1895 [= Psilopa roederi Girschner, 1889]) is resurrected from synonymy with Psilopa and the following four new synonyms of Diasemocera are proposed: Domina Hutton (Type species: Domina metallica Hutton), Discocerinella Mercier (Type species: Discocerinella omonvillea Mercier, 1927 [= Notiphila pulicaria Haliday, 1839]), Trimerinoides Cresson (Type species: Trimerina adfinis Cresson) and Helaeomyia Cresson, 1941 (Type species: Psilopa petrolei Coquillett). Diasemocera is placed close to the genus Trimerina and it differs from the highly specialized Psilopa and related genera by the number of pseudotracheae (6–7), vestiture of prosternum, and characters of the male terminalia. A diagnosis of Diasemocera and a key for the species are provided. All 24 included species, previously placed in Psilopa, are combined with Diasemocera for the first time. One of the species is the petroleum fly, now named Diasemocera petrolei (Coquillett, 1899) and it belongs to Psilopini. Six species are treated as new synonyms; three in Diasemocera: Diasemocera nigritella (Stenhammar, 1844) = D. glabricula (Fallén); D. aethiopiae (Canzoneri, 1987) = D. nervimaculata: Becker (1910); D. punica (Canzoneri, 1991) = D. fratella (Becker, 1903), and four in Psilopa: Psilopa rutilans Canzoneri & Meneghini, 1972 = Psilopa radiolata (Becker, 1903) = Psilopa clara (Wollaston, 1858); Psilopa pappi Canzoneri and Meneghini, 1975 = Psilopa stackelbergi Nartshuk, 1970; Psilopa nettmanni Stuke, 2014 = Psilopa thora Dahl, 1973.