Neotropical fishes present a high chromosome diversity showing a wide diploid number variation range, including different levels of ploidies, sex chromosomes, chromosome supernumeraries, and several cases of polymorphisms, related particularly to heterochromatin and NOR sites. Two main general trends of chromosome diversification can be observed among neotropical fishes. First, several fish groups show a chromosome evolution relatively divergent from the point of view of the karyotypic macrostructure. Sister species show conspicuous differences in karyotype structure and most often also in the number of chromosomes. On the other hand, there are fish groups in which chromosome evolution has been shown to be less divergent, and in this case whole families or even groups of families may share a common karyotype structure and equal number of chromosomes. Several fish groups appear conservative also with respect to the NOR bearing chromosomes. In this case, NOR chromosome location is invariable among species. In contrast, several other groups present wide NOR variability. Sister species may show quite diverse chromosomes bearing nucleolar organizing regions. The NOR and heterochromatin relationship is also very diverse among fishes and this may indicate organizational differences involving these chromosome segments. Thus, neotropical fish fauna presents great chromosome variability, verifiable also by NOR studies.