More than 70 dinoflagellate cyst taxa have been encountered in a palynological study of 34 samples from part of the Neogene section of Hole 1151A, Ocean Drilling Program (ODP) Leg 186 off the Sanriku Coast of northern Japan, northwestern Pacific. According to the diatom biostratigraphy, the age of the studied section ranges from the latest early Pliocene down to the earliest middle Miocene. On the basis of the present analysis, 17 biohorizons and possible biohorizons are discriminated as potential biostratigraphic markers. Seven acme events are also listed. The biohorizons show overall consistency with the stratigraphic ranges of species given by earlier studies of onshore sections in northern Japan, which indicates regional implications of those biostratigraphic events. In addition, dominance of protoperidiniacean species in the late late Miocene to early Pliocene assemblages from the present hole compares with a similar dominance of the same age reported previously in the Bering Sea and northern North Pacific. This implies establishment of nutrient-rich surface water in the wide region of the northwestern to north Pacific in the late Miocene.