Karyotype studies were carried out on two fish species of the family Gymnotidae. In five samples ofGymnotus carapofrom different localities chromosome number varied from 2n= 42 to 2n= 54. Specimens from the Amazon Basin had 2n= 42 (Belém—State of Pará) and 2n= 48 (Humaitá - State of Amazonas); from the Paraná Basin, 2n= 52 (Brotas—State of São Paulo) and 2n= 54 (Botucatu—State of São Paulo); from the East Basins, 2n= 54 (Miracatu—State of São Paulo). These differences in diploid number were accompanied by differences in the number of acrocentric chromosomes present in the complement. Only one sample ofGymnotus sp., from Miracatu, was analyzed. Here the diploid number was 2n= 52 and the complement was similar to that ofG. carapofrom Brotas.