Cytogenetic characterization of two freshwater sciaenid species from the genus Plagioscion (P. squamosissimus and Plagioscion sp.) was obtained for the first time. Giemsa staining, Ag-NOR and C-banding revealed that both species presented 2n = 48 chromosomes (almost all acrocentric). Single NORs and heterochromatin were found mainly at the pericentromeric position. Karyotypic formulae and NOR location proved to be valuable in showing both interspecific and intraspecific differences. All chromosomes were acrocentric in P. squamosissimus. NORs were located at proximal positions on the long arms of the last chromosome pair of the complement, and were heteromorphic due to size differences. Such heteromorphic NORs seem to be associated with each population sampled. Plagioscion sp. presented two cytotypes: cytotype a (2M + 46A) and cytotype b (48A). In both cytotypes, NOR-bearing chromosomes were located at the proximal position on the long arms of the first chromosome pair of the complement. However, NOR-bearing chromosomes were metacentric in cytotype a and acrocentric in cytotype b.
