Dinoflagellate cysts from the uppermost lower Pliocene through middle Miocene of Leg 186, constrained by a diatom biostratigraphy, are shown to have excellent potential for biostratigraphy and paleoenvironmental reconstruction. We present 17 biohorizons and possible biohorizons and several acme events based on an abundant and well-preserved record. Because these biostratigraphic events compare favorably with published data from onshore sections in northern Japan, a basically consistent floral development is recognized through the northern Japan area, including both the forearc and backarc sides, leading to potentially useful regional stratigraphic correlation. More detailed study of dinoflagellate cysts from the present section should serve as a standard reference for Pliocene through middle Miocene correlation and dating by dinoflagellate cyst biostratigraphy in northern Japan and its surrounding areas.