An X 1 X 2 Y sex chromosome system is reported for the first time in Gymnotus sp. The chromosome number observed was 2nϭ40 (14 M-SMϩ26 ST-A) in females and 2nϭ39 (15 M-SMϩ24 ST-A) in males, with the same fundamental number in both sexes (FNϭ54). The multiple sex chromosome system might have been originated by a Robertsonian translocation of an ancestral acrocentric Y-chromosome with an acrocentric autosome, resulting in a metacentric neo-Y chromosome observed in males. Single NORs were detected

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... on the short arm of a middle-sized acrocentric chromosome pair. Constitutive heterochromatin was observed in the pericentromeric regions of several chromosome pairs, including the neo-Y chromosome and the NOR carrier chromosomes. The DAPI/CMA 3 stain revealed that all the pericentromeric heterochromatin are AϩT rich whereas the NORs were associated with GϩC rich base composition. The possible ancestral condition characterized by an undifferentiated Y-chromosome from all the Gymnotiformes fishes is discussed.