Chromosomes of Moenkhausia sanctaefilomenae (10 males and 20 females) collected from a tributary of the Tiete River (Botucatu, S.P. Brazil) were examined using kidney and testicular cells. Both males and females presented 2n=50 chromosomes and 1 to 8 small supernumerary microchromosomes. C-banding demonstrated positively stained heterochromatic blocks in almost every chromosome and a pattern of interstitial bands located in the same position in relation to the centromere on the long arm of a large number of chromosomes. Two large NORs were detected in pair 19 of all the silver-stained metaphases; zero to 6 additional small NORs were detected in other chromosomes. A general survey of the occurrence of supernumerary chromosomes in fishes is presented.