The calcareous nannoplankton of cores from the experimental Mohole drilling indicate that Pliocene and Miocene strata were penetrated, with middle Miocene overlying the basalt in which drilling ended. Coccoliths and discoasters are the common forms, although few in number of species, and some common genera are absent. Similarity in this respect to the late Tertiary of California suggests a relatively cold current extending S. beyond California during the late Tertiary, and comparable to the present California Current. Biostratigraphic correlations are consequently less well based than for warmer water regions. Of 20 species described, 6 are new species of the genus Discoaster (Discoaster bollii, D. exilis, D. hamatus, D.. kugleri, D. surculus, D. variabilis) and 2 are new species assigned to the new genus Catinaster (Catinaster calyculus, C. coalitus). Interpretations of age relations are based on the occurrence of these species at late Tertiary localities of Italy, Trinidad, California and elsewhere.