The foraminiferal samples dealt with herein were collected from the control section of the Nakatsu formation exposed on the River Sagami., Microbiostratigraphically, the Nakatsu can be subdivided into two zones, the lower, the Quinqueloculina bicostata and the upper, the Rotalia sagamiensis., Many of the species from the Quinqueloculina bicostata zone, which includes five zonules, are known in shallow waters from the northern part of Japan, and this zone contains such megafossils as Umbonium arenarium, U, obsoletum and U., conglomeratum., Rotalia sagamiensis zone which includes four zonules, is characterised by the abundance of Rotalia, Nodosaria, Burimina, Bolivina, and Gyroidina, and contains many characteristic species such as Planuleria japonica, Parafrondicularia japonica, Plectofrondicularia nakatsuensis n., sp., and Robulus submamilligerus var., etc., ., Judging from the above criteria, the Lower Nakatsu is considered to have been deposited in shallow and cold waters and the Upper Nakatsu in deeper and warmer water than the Lower., The writers think that the Lower Nakatsu is the upper part of the lower Pliocene (H1b) and the Upper Nakatsu is the lower part of the middle Pliocene (H2).,