Abstract
I nthe present study the karyotypes and the number and position of nucleolus organizer regions (NORs) of four species of Characiformes are described. The analyses showed that Acestrorhynchus lacustris had 2n=50 chro- mosomes (8M+34SM+6ST+2A), Oligosarcus longirostris had 2n=50 chromosomes (2M+20SM+10ST+18A), Oli- gosarcus cf. paranensis from Keller and Mouraorivers had 2n=50 chromosomes (2M+26SM+8ST+14A), and Rhaphi- odon vulpinus had 2n=54 chromosomes (32M+12SM+8ST+2A). The number of NOR-bearing chromosome pairs ranged from one to three among the species studied. The results available show that there is no variation in the dip- loid number within the genera analyzed. However, clear differences related to the karyotypic formulae and the number and position of Ag-NORs were found even in the comparison between populations. The possible relation- ships between the genera analyzed and other Characiformes are discussed.
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