**3. Conclusions**

Molecular studies with the multigene family 5S rDNA in electric fish (Gymnotidae) have advanced a lot in recent years. The chromosomal location and distribution have been particularly interesting, since all species of the genus *Gymnotus*, analyzed so far, show great variability. Its distribution on the chromosomes of species with 2n = 54 shows to be a biogeographic marker, suggesting that the speciation of this group was recent due to migratory expansion. On the other hand, the species analyzed in the present study showed repetitive sequences composed of microsatellite replications species-specific, present around the centromeres. Although the location of the heterochromatin in *Gymnotus* was conserved, they had different constitutions and could represent important evolutionary markers for cytotaxonomic studies of this group.
