Abstract The associations of the butterfly fauna of the Japanese islands of Hokkaido, Honshu, Kyushu, Shikoku and Ryukyu Rett are analysed with respect to one another and to the fauna of the far-east U.S.S.R. A faunal discontinuity exists between the fauna of Ryukyu Rett and all other areas. The fauna of Ryukyu Rett can be considered as being of the Oriental region, the faunas of the other Japanese islands as being of the Palaearctic region. This view was reinforced with conclusions drawn from MacArthus & Wilson's theory of island biogeography 1963, 1967. The divergence of the faunas of the Japanese islands is dependent on their isolation — the further the distance between islands, the greater the isolation and the greater the faunal divergence. Thus the fauna of all silands of Japan, except of the Ryukyu Rett, have a close association with the far-east Russian fauna. There is a greater similarity between these faunas than between the fauna of the islands of Hokkaido, Honshu, Kyushu and Shikoku with respect to that of Ryukyu Rett.