The scorpion Tityus serrulatus Lutz & Mello, 1922 (Buthidae) is endemic to Brazil and is considered the most toxic South American scorpion species. The ample distribution of this species and its occurrence in urban areas has been attributed primarily to the parthenogenetic mode of reproduction. For a long time, only all-female T. serrulatus populations were known. Male specimens are very rare and have been recently discovered in natural environments within the Brazilian semi-arid zone. In the present study, we investigated cytogenetically bisexual populations of T. serrulatus, and compared the data with those available for parthenogenetic specimens. The diploid number (2n=12) and the general configuration of the chromosomes in male and female specimens were similar to those recorded for parthenogenetic populations. Male postpachytene cells contained chromosomes in a parallel arrangement with no differentiation of the bivalents, indicating the absence of heteromorphic sex chromosomes. However, the number of 28S rDNA sites varied between male and female specimens. Females invariably presented only one rDNA cistron, while all the males had two. This finding seems to indicate the existence of a sex-specific heteromorphism in T. serrulatus, in which the females are heterogametic and the males are the homogametic sex.