Cytogenetic analysis of Trichomycterus davisi, collected from the Iguaçu River basin, has shown a diploid number of 2n = 54 chromosomes. However, we observed intra-individual numerical polymorphism in a T. davisi specimen. There were three cell populations with diploid number 2n = 54 (40M + 12SM + 2ST), 2n = 55 (40M + 12SM + 2ST + 1M) and 2n = 56 (40M + 12SM + 2ST + 2A) chromosomes. This variation was attributed to a probable post-zygotic non-disjunction of a metacentric chromosome of small/middle size, followed by centric fission, which originated in this individual.
