Diatoms were studied from upper Miocene through Recent sediments recovered at Hole 1149A in the western Pacific during Ocean Drilling Program Leg 185. The upper 118.2 meters (Unit I) consist of upper Miocene to upper Pleistocene carbonate-free clay with common volcanic ash particles and siliceous microfossils. Seventy-five samples were examined from Unit I for diatoms. Fifty samples ranging from early Pliocene to late Pleistocene in age, contained 138 diatom species, and were attributable to the Neodenticula kamschatica through Neodenticula seminae Zones of the North Pacific diatom zonation, and to the Nitzschia jouseae through Pseudoeunotia doliolus Zones of the Equatorial Pacific diatom zonation. Patterns of diatom abundance in Unit I correlate well with the diatom events of Barron (1998). The data seem to show Barron's (1998) Events C and D and increases in diatom sedimentation rate in the late Pleistocene not specifically named by Barron (1998), but shown in his data. Inconsistencies of species ranges compared to published data suggest either extensive reworking or biogeographic control of first and last occurrences for some zonal indicator species. The occurrence of displaced Miocene, neritic, and benthic taxa strongly suggests aqueous or eolian transport of materials from coastal environments and/or land-based outcrops. Diatom temperature (Td) and paleoclimate ratios (Twt and Td (sub f) ) computed for Hole 1149A were compared and showed general agreement with one another as to glacial/interglacial events in the Pliocene and Pleistocene. Sea surface temperatures calculated for Hole 1149A also correlated with major climatic events in the Pliocene and Pleistocene.