It is only since the advent of the Deep Sea Drilling Projet (DSDP) and the wider availability of older Cenozoic deep-sea core material that large-scale pre-Quaternary paleoenvironmental studies using marine microplankton could be attempted. The present study is one of the first attempts at mapping the early Cenozoic (65--24 m.y. B.P.) spatial and temporal distributions of calcareous plankton of the Atlantic Ocean using the DSDP cores and reconstructing the paleoclimatic history of the region from the resultant patterns. The early Cenozoic biogeographic patterns of calcareous nannoplankton (coccoliths and discoasters) and planktonic foraminifera have been delineated on the basis of quantitatively defined assemblages. These patterns show (1) that latitudinal differentiation among the calcareous planktonic groups existed during most of the early Cenozoic, with the exception of the earliest Paleocene (65--64 m.y. B.P.), when planktonic foraminiferal assemblages were essentially homogeneous through all latitudes, showing little provinciality, and when the floral gradients exhibited by nannoplankton were more closely related to near-shore and open ocean conditions and (2) that there have been major changes in the distributional patterns of both groups: those related to major latitudinal shifts with time and those due to evolution and disappearance of various forms. As is true in the present-day ocean, the early Cenozoic patterns are considered to be controlled mainly be the latitudinal thermal gradient (climate). Thus the temporal oscillations in the assemblages are interpreted as being caused by major climatic fluctuations. Four marked cooling episodes are recorded within the early Cenozoic Atlantic Ocean: those during the middle Paleocene (60--58 m.y. B.P.), the middle Eocene (46--43 m.y. B.P.), the earliest Oligocene (37--35 m.y. B.P.), and the middle Oligocene (32--28 m.y. B.P.). A particulary marked warming episode occurred during the late Paleocene-early Eocene (54--51 m.y. B.P.), and a second, less prominent warming trend began in the latest Oligocene (28 m.y. B.P.) and continued into the early Miocene.