Abstract
Were citogenetically analyzed 83 fish species from the Paraná-Plata river basin. Diploid numbers range from 36 to 102 chromosomes and three modal numbers were determined 2n=54, 2n=48 and 2n=56 for Characiformes, Perciformes and Siluriformes, respectively. Supernumerary chromosomes were observed also, as well as polymorphic B chromosomes, microchromosomes and systems of sex chromosomes (ZZ/ZW). The present data will be useful for cytotaxonomic and evolutionary studies.