Planktonic foraminifera from 91 samples of ODP Sample 170-1043A-18X through 30X, drilled on the lowest continental slope of the Middle American Trench off the Nicoya Peninsula, the Pacific coast of Costa Rica are examined. The sediments of Samples 170-1043A-18X through 30X-CC are included in Zones N17 to N23, Late Miocene to Pleistocene in age. Planktonic foraminiferal components are rich warm-water specimens in general. Cold-water planktonic foraminifera are, however, included at a high percent in the Core 27X-CC, of which sediments are assignable to N19 at ca. 4 Ma, suggesting an influence of the closing of the Central American Seaway.