An assessment of polymorphism and the level of differentiation in insular and mainland populations of the field mouse was carried out. Specimens of four islands in the Peter the Great Bay of the Sea of Japan and four regions on the mainland of the of the Russian Far East South were tested. Nuclear markers of 5 microsatellite loci were studied. The results demonstrate higher allelic diversity in the mainland part of the range, with a significantly higher level of differentiation in the insular part of the range.