Some 25 polychaete species common to Asiatic and American coasts are recorded in south-boreal and subtropical waters of the Pacific; they are absent in both northern cold regions (Bering and Okhotsk seas) and in tropical zones. Examples are also given of analogous, closely related twin species, and subspecies. Such disjunct amphipacific ranges of these species are probably caused by ancient changes in climatic conditions. The possibility of modern oceanic currents having some influence on this is also not excluded. The author suggests a study of life cycles of amphipacific species in the extremes of their habitats.