Dissomphalus is a cosmopolitan genus of Bethylidae and has 269 Neotropical species divided into 32 species-groups, mostly defined by the genital and the tergal process structures. Dissomphalus rectilineus and D. concavatus are sympatric species in the ulceratus species-group. Members of the species-group share many similarities in the morphology of the head, hypopygium, tergal process and genitalia, but may be distinguished by the structure of the hypopygium. Previous studies have found intermediate structures of the hypopygium in the sympatric areas and raised questions about the distinctiveness of these two species. We sequenced 340 bp of the mitochondrial gene cytochrome oxidase I of 29 specimens from Brazil and Paraguay, calculated the genetic divergence among specimens, and recovered the phylogenetic relationships between taxa. In addition, we compared the morphology of the hypopygium to evaluate its use as a species-specific diagnostic character using the genetic divergence values. We recovered three well-supported monophyletic groups (intraclade divergence from 1.3 to 13.4%) and three hypopygium morphologies associated with each clade, two of them associated with D. rectilineus and D. concavatus (as described in the literature); the third one is new, not associated with any known species. The divergence between the D. rectilineus and D. concavatus clades was 19%, while the third clade is divergent from each species by 19–20%. If fully described, the hypopygium shape associated with the COI sequence will represent an extremely promising approach to the diagnosis of Dissomphalus species.