Karyotypes of five males and four females of Eurasian (Siberian) flying squirrel,Pteromys volans orii, were analyzed with G-, C-, Ag-NOR staining methods. Fluorescencein situhybridization (FISH) was applied to the location of 18S and 28S ribosomal RNA genes. The diploid chromosome number of this species was 2n= 38 with the arm number (NF) of 68. C-bands were located at the centromeric region of all chromosome pairs. Nucleolus organizer regions (NORs) appeared in the short arm of chromosome 17, and the location of Ag-NORs was indistinguishable from the site of the rRNA genes. The size variation of NORs was observed in each individual, indicating heteromorphism of NORs in this species.