Cytogenetic studies of genus Pimelodus (P. maculatus, P. ornatus, P. absconditus) occurring in the River Paraná were carried out. The 3 species analyzed resulted in the number 2n=56 chromosomes with differences in the formulae karyotypic. The nucleolus organizer regions (NORs) showed heterochromatic and CMA₃+in the 3 species studied. In P. maculatus and P. absconditus this region was localized in terminal position on the long arm of a subtelocentric chromosome pair, while in P. ornatus the region lies on the short arm of a subtelocentric pair. Analysis of C-band pattern showed blocks located in centromeric and telomeric position in the 3 species and interstitial in P. maculatus and P. absconditus. Karyotypical comparisons with regard constitutive heterochromatin and NOR banding demonstrated a greater similarity between the 2 species.