The karyotype and the Ag-NOR location of a sample of Trichomycterus diabolus collected in the córrego Hortelã (Botucatu, São Paulo, Brazil) are described. The species exhibited 2n=56 chromosomes (42 metacentrics, 12 submetacentrics and 2 subtelocentrics) and the nucleolus organizing region located near to the centromere on the long arm of the largest metacentric pair. The presence of 2n=56 chromosomes in T. diabolus is an interesting characteristic since until now all cis-Andean species karyotyped presented 2n=54 chromosomes while almost all trans-Andean species presented different diploid numbers. The possible origin of this unexpected karyotypic form is discussed.
Trichomycterinae; karyotype; chromosome evolution; fish cytogenetic