A quantitative phytoplankton study in Korean waters was commenced in 1964 as a part of the primary production studies of Koreans seas, and it was continued with the cruises for Cooperative Studies of the Kuroshio(C.S.K) in 1965-1968. Phytoplankton samples were taken by dipping about 500ml of sea water from the surface, and then fixed by ading neutralized formlin. This report deals with the results obtained during 1965-1966. I examined a total of 298 samples of surface phytoplankton collected in the wate neighboring Korea in the above-mentioned period, and detected 147 species of diatoms and 22 species of dinoflagellates. Among them 123 species of diatoms and 18 species of dinoflagellates occured in the Japan Sea region, 133 species of diatoms and 11 species of dinoflagellates occured in the Korea Strait region, and 49 species of diatom and 8 species of dinoflagellates occured in the Yellow Sea region. And thd phytoplankton standing crops are dept in a fair abundance in the Japan Sea area all the year round, and are poor in the Yellow Sea area. The seas surrounding Korea are divided into seven regions by the planktological characteristics; northern and southern parts of the Japan Sea, eastern, western and southern parts of the Korea Strait, southern and northern parts of the Yellow Sea. The representative of the phytoplankton community in each sea region is generalized as follows; northern part of the Japan Sea is dominant with Chaetoceros group, southern part of the Japan Sea is dominant with Chaetoceros group and Skeletonema costaum, eastern part of the Korea Strait is dominant with Chaetoceros group and Pleurosigma sp., southern part of the Korea Strait is dominant with Chaetoceros group and Rizosolenia group, western part of the Korea Strait is most poor in phytoplankton, southern part of the Yellow Sea is dominant with Pleurosigma sp. and Coscinodiscus group, and northern part of the Yellow Sea is dominant with Pleurosigma sp. and Eucampia zoodiacus. Chaetoceros curvisetus, Leptocylindrus danicus, Pleurosigma normanii, Thalassionema nitzschioides, Thalassiothrix flauenfeldii appeared all the year round in the neighboring sea of Korea. There were 24 species (18 species of diatoms and 6 species of dinoflagellates) of the pecuriar phytoplankton in the Japan Sea, 27 species (25 species of diatoms and 2 species of dinoflagellates) of that in the Korea, and 7 species (5 species of diatoms and 2 species of dinoflagellates) of that in the Yellow Sea, respectively.