Distributions of microplankton in two different regions of the East China Sea were studied based on the plankton samples collected by R. V. Umitaka Maru of Fisheries University of Tokyo, Japan in September, 1975. The abundance distribution and species composition of the two areas were interpreted with their temperature-salinity properties. The two areas aye characterized by differences in the depth of thermocline, the nature of surface and bottom waters, and the abundance and species composition of microplankton.