In the present study, we analyzed 20 specimens of Bryconamericus aff. iheringii (12 males and 8 females), collected from the Três Bocas Stream, a tributary of the Tibagi River/PR. All individuals analyzed presented 2n=52 and different karyotypic formulae resulting in 4 cytotypes: cytotype I, with 12m+16sm+10st+14a and FN=90; cytotype II, with 14m+18sm+10st+10a and FN=94; cytotype III, with 10m+24sm+6st+12a and FN=92; and cytotype IV with 10m+14sm+8st+20a and NF=84. Such structural karyotypic variations among Bryconamericus aff. iheringii specimens are probably due to the occurrence of chromosomal rearrangements, such as pericentric inversions. In some individuals, nucleolus organizer regions (AgNORs) were observed on the short arm of a single submetacentric chromosome pair. In other individuals of this same population, an interindividual variation from 2–5 NOR-bearing chromosomes with markings located on chromosomes of different types and size was detected, along with AgNOR size heteromorphism between the homologous chromosomes in a metacentric pair. Thence, this population of Bryconamericus aff. iheringii from the Três Bocas Stream seems to show the occurrence of both multiple and single NOR systems, and also the presence of different karyotypes (cytotypes), which might suggest the occurrence of 2 different species of Bryconamericus living in sympatry in that location. The data discussed here and those presented in the literature show the need for a taxonomic revision in this group of fish.