Three biostratigraphic zones in the late Pliocene-Pleistocene sediments of the North Pacific are defined using diatoms with limited ranges. The extinction of one species is examined; in five out of six cores it occurs at the lower boundary of the Jaramillo event (.87 to .89 m.y.B.P.). Diatom temperature values (Td values) based on warm and cold water species are calculated so that low Td values indicate lower temperature. The Td values reveal two major divisions within the Pleistocene: 2 m.y. to 700,000 years B.P. is a time of relatively stable Td values; 700,000 years B.P. to the Recent has a lower average Td value than the 2 m.y. to 700,000-year interval and shows greater fluctuations in the Td curve. One core contains nine Td value minima in the last 700,000 years.