The Kuril Island Chain, which extends 1200 km in a northeast direction from Hokkaido (Japan) towards the shores of the Kamchatka Peninsula (Russian Federation), unites the boreal flora of the southern Kamchatka with the temperate flora enriched with subtropical elements from the lower elevation belts of Hokkaido. The gradual shift in the liverwort flora from the north to the south is analysed and described. It is shown that the border between East Asian and Circumboreal floristic regions (based on an analysis of liverworts) should run either along the Izmena Straight between Hokkaido and Kunashir Islands, or along the Ekaterina Straight between Kunashir and Iturup Islands, the former being a better-justified position.