The chromosomal location and size of the 18S–28S ribosomal RNA (rRNA) gene cluster was examined by fluorescence in situ hybridization (FISH) and AgNO₃ banding in three species of the Asian salamander family Hynobiidae: Hynobius quelpaertensis (2N=56), H. tsuensis (2N=56), and Onychodactylus koreanus (2N=78). These species have karyotypes that contain numerous microchromosomes and are thought to resemble the karyotype of the most recent common ancestor of all living caudate amphibians. In the Hynobius species FISH signals representing 18S–28S rDNA sequences were found on multiple single chromosomes as well as on a single homologous pair of microchromosomes (no. 23). Co-localization of AgNO₃ bands was found on the same pair of microchromosomes indicating that this site represents transcriptionally active NORs (Ag-NORs). Onychodactylus showed FISH signals on an unidentified pair of microchromosomes, but these did not show AgNO₃ bands, which were found only on several macrochromosomes and microchromosomes. These results suggest that 1) an active NOR located in a single pair of microchromosomes may have been shared by the common ancestor of hynobiids; 2) FISH revealed the presence of rDNA sequences in a similar-looking pair of microchromosomes in Onychodactylus, although the site seems to be Agnegative; and 3) the similarity of chromosomal localization of the rDNA sequences between Onychodactylus and Hynobius suggests that the location of 18S–28S rDNA sequences on a pair of microchromosomes may be a common feature in the asymmetrical bimodai (AB) karyotypes of hynobiid salamanders.