An analysis of the nucleolar organizer region (NOR) by silver staining of five species of the order Gymnotiformes, Gymnotus carapo, Apteronotus albijrons, Sternopygus macrurus, Eigenmannia virescens, and Eigenmannia sp., is reported. The five species presented only one pair of homologs bearing NORs. In G. carapo, A. albijrons, and S. macrurus the silver-stained NORs were present as small black dots over the chromosomes. In E. virescens, the short arm of one of the largest chromosome pairs was entirely stained with silver. Eigenmannia sp. presented an intraspecific variability of the NOR-bearing pair and an increase in the length of this region, the larger one being about six times the size of the smaller. These findings suggest that an increase in the number of ribosomal genes in the NORs of certain species occurred during the evolution of this group.