Several diatom datum levels are proposed for latest Miocene/earliest Pliocene sediments in the equatorial and north Pacific. These datum levels are tied directly to the magnetostratigraphy and may be summarized as follows: 1) Last occurrence of Denticulopsis hustedtii —middle of Chron 10 in the equatorial Pacific. 2) First occurrence of Thalassiosira burckliana —lower part of Chron 9 in the equatorial Pacific. 3) First occurrence of Coscinodiscus nodulifer var. cyclopus —middle of Chron B in the equatorial Pacific. 4) Last occurrence of Thalassiosira burckliana —lowest part of Chron 7 in the equatorial Pacific. 5) First occurrence of Rossiella praepaleacea —lower part of Chron 6 in the central Pacific. 6) First occurrence of Nitzschia miocenica var. elongata —lower part of Chron 6 in the equatorial Pacific. 7) Last occurrence of Rossiella praepaleacea —upper part of Chron 6 in the equatorial Pacific. 8) Last occurrence of Nitzschia miocenica var. elongata —middle part of Chron 5 in the equatorial Pacific. 9) First occurrence of Denticulopsis kamtschatica —lower part of Chron 6 in the high latitude North Pacific. 10) Last occurrence of Rouxia californica —middle part of Chron 6 in the high latitude North Pacific. Within the context of a more refined biostratigraphy and improved correlation between high and low latitudes, we can trace the latitudinal changes in distribution of selected diatom species as a function of time. Actinocyclus ingens is seen as a cosmopolitan species in the middle Miocene and by the Late Miocene it has retreated to the higher latitutes of the north and south Pacific. Denticulopsis kamtschatica, on the other hand, is shown to make its first appearance in the high latitude north Pacific at about 6.3 Ma and extend its range southward during the succeeding 1.5 Ma.