Cluster analysis of bivalve species recorded in Cenozoic deposits in Sakhalin Island, western Kamchatka, Hokkaido, and California was used to determine geological age of the modem North Pacific biogeographic region and its constituent subregions (Japan-Mandchurian, Beringian, and Oregon-Sitkan). The North Pacific region developed during the Paleogene-Neogene transition due to Drake Passage opening to deep-water movement, formation of the deep-water Antarctic Circumpolar Current, and the change in climate from greenhouse to psychospheric. Differentiation of the three subregions within the North Pacific Region seems to have occurred in late Miocene-early Pliocene, about 5.6 millions years ago and was probably due to the flooding of the Bering Land Bridge and development of the present configuration of circulation in the North Pacific. In the Northwest Pacific, during Paleogene and early Neogene, the faunal diversification occurred more rapidly and was more extensive than in the Northeast Pacific.