A probe containing most of the transcription unit of the major ribosomal genes of Xenopus laevis has been used to localize the rDNA cluster of European eel (Anguilla anguilla) by fluorescence in situ hybridization (FISH). Only on NOR-bearing chromosome pair has been found. The results obtained with this technique confirm the size polymorphism of the NOR revealed previously by other classical cytogenetic techniques.