Some species ofHypostomusfrom the Upper Paraná Basin (SP) were studied cytogenetically. The diploid chromosome number ranged from 2n= 68 to 2n= 80, with a considerably variable karyotypic structure between species, but no chromosomal differences were observed between males and females. The nucleolar organizer regions (NORs) were also quite variable both intra- and interspecifically in terms of number and size. In general, constitutive heterochromatin was not very abundant and usually located in small blocks. The data suggest that some chromosomal rearrangements, as the robertsonian ones and pericentric inversions, were important for the karyotypic evolution ofHypostomus.These fishes also appear to be a good material for cytotaxonomic studies.