The karyotypes of six species of Acestrorhynchinae (Acestrorhynchus alus, A. lacustris, Oligosarcus hepsetus, O. jenynsii, O. macrolepis and O. pinloi) and of one species of Cynopotaminae (Galeocharax knerii) were studied. The six Acestrorhynchinae species have 2n= 50, while Galeocharax knerii has 2n= 52 chromosomes. Some chromosomal characteristics were detected which permit establishing some karyotypic relationships among the different species investigated. Thus, among the Acestrorhynchinae, the four Oligosarcus species are relatively more related to one another than the two Acestrorhynchus species, at least with respect to the cytogenetic data considered. On the basis of the methods used, no sex chromosome heteromorphism was detected in the species for which a comparative study between male and female specimens was possible.