It is difficult to make reliable sex associations in several species of Hymenoptera due to sexual dimorphism. Only a few species of the flat wasp genus Dissomphalus Ashmead, 1893 have had their sexes associated, since females are rarely collected and differ morphologically from their conspecific males. Collecting couples during their phoretic copula is difficult, but it is the most reliable way to associate the sexes in Dissomphalus. Herein, we associate the sexes and discuss the mating system based on couples of six Dissomphalus species collected during copulation: D. bisserratus Azevedo, 1999, D. connubialis Evans, 1966, D. rettenmeyeri Evans, 1964, D. simulatus (Evans, 1969), comb. nov., D. mendicus Evans, 1969, and D. firmus Redighieri & Azevedo, 2006. The females of the latter three are described for the first time. The distribution of two species is expanded. Apenesia simulata Evans, 1969 is transferred to Dissomphalus and is now D. simulatus comb. nov. Finally, we briefly discuss the genital structures used during phoretic copulation, analyzing the male and female genitalia in copulation. During copulation, the mating female is attached to the male only by the genitalia and is apparently grasped by muscles attached to the male's metasoma.