The genetic variation of the forest dormouse Dryomys nitedula (Pallas, 1779) from isolated populations of Russian Plain and the Caucasus was investigated using cytochrome b gene (cytb). The genetic distance calculated between these populations of forest dormouse was 9.94 %, which corresponds to the typical distance between biological species of mammals. The genetic distance of cytb between Western and Central Caucasus forest dormouse populations is also significant, 6.0 %. Probably, there was a long-term isolation of European and Caucasian areas of D. nitedula during the whole Pleistocene.