In order to stablish new taxonomic characters for the genus Opisthcidius a detailed comparative study of the male external genitalia was performed in O. rubropictus (H. Shaeffer, 1848), the type species, and O. pertinax (Breddin, 1903), mantioned as predators of triatomines. The most representative phallic structures were: the median process of pygophore, the phallosoma struts and the vesica process.