The phytoplankton taxonomic composition of the subsurface chlorophyll maximum layer (SCM) was compared with that of the surface layer in July, Sept. and Nov. 1979 in the western North Pacific Ocean. Besides the use of chemical fixatives for preservable phytoplankton, serial dilution culture method was employed for non-preservable flagellates and monads, i.e., fragile forms. The SCM was characterized by a high species diversity of preservable phytoplankton and by the numerical dominance of fragile forms. Among the fragile forms, Micromonas and Ochromonas were dominant. This dominance is consistent with their viability at low light intensity. Cluster analysis revealed that, for blue-green algae, coccolithophorids, silicoflagellates and diatoms, a marked difference existed in species assemblages between the SCM and the surface, but there was no distinct difference in dinoflagellate assemblages. The difference between both layers is discussed, as related to the mechanisms of formation of the SCM.