RESUMEN
We cytogenetically characterized three species of Heptapteridae (Pimelodella sp, Pimelodella taenioptera, and Imparfinis schubarti) by investigating the distribution of constitutive heterochromatin and nucleolar organizer regions by silver nitrate impregnation (Ag-NOR) and fluorescence in situ hybridization. Pimelodella sp showed had a diploid number (2n) = 46 chromosomes, 26m + 10sm + 10st, and FN = 92; P. taenioptera, 2n = 52 chromosomes, 26m + 22sm + 4st, and FN = 104; and I. schubarti, 2n = 58 chromosomes, 28m + 28sm + 2st, and FN = 116. The two Pimelodella species had Ag-NORs sites on the submetacentric pair 14, located on the short arm in terminal position. In I. schubarti, the Ag-NORs sites were in an interstitial position on the long arm of the metacentric pair 1. C-banding revealed that Pimelodella sp contained a small amount of constitutive heterochroma-tin, whereas P. taenioptera contained a higher number of heterochromatic regions, in the pericentromeric, interstitial, and telomeric positions. I. schubarti had markers in centromeric and telomeric regions of a few chromosomes, and a large pericentromeric block on pair 1. Fluorochrome chromomycin A3 (CMA3) staining revealed positive signals on pair 14 in both Pimelodella species. Treatment with 4ê,6-diamidino- 2-phenylindole (DAPI) revealed no markings in P. taenioptera, but an interstitial marking on the long arm of pair 14 in Pimelodella sp. In I. schubarti, positive signals of CMA3 were detected in the first pair, but negative signals were detected for DAPI staining. These results con-tribute to the karyotypic description of the less-studied species in the Brazilian Midwest.