We examined the planktonic foraminiferal biostratigraphy of Pliocene to Miocene obtained from Sites 1150 and 1151 of ODP Leg 186. We detected 16 stratigraphically important taxa of planktonic foraminifers from these sites. In spite of the stratigraphic discontinuities of these species, owing to calcite dissolution, the following eight planktonic foraminiferal biohorizons were recognized in the same descending stratigraphic order at both sites: (1) the LO of N. asanoi; (2) the LO of G. puncticulata; (3) the FO of G. inflata (modern form); (4) the FO of N. asanoi; (5) the LO of G. ikebei; (6) the FO of G. ikebei; (7) the LO of G. nepenthes; and (8) the LO of G. dehiscens. According to the stratigraphic relationship between foraminiferal and diatom biohorizons (Maruyama and Shiono, this volume), the eight foraminiferal biohorizons seem to be synchronous in the Japan Trench region. The LO of N. asanoi, which occurs between Samples 186-1150A-14X-5, 42-44 cm, and 16X-1, 40-44 cm (122.8-136.0 mbsf), and between Samples 186-1151A-6R-1, 40-43 cm, and 7R-3, 40-44 cm (113.0-125.9 mbsf), occurs just below the Pliocene/Pleistocene boundary.