Monthly observations of phytoplankton and physico-chemical properties of sea water were made from October, 1977 to December, 1977 and from June, 1978 to August, 1978 at four stations in Cheonsu Bay. 141 taxa of phytoplankton(diatoms and dinoflagellates) representing 17 families, 40 genera have been identified in this study, and 11 taxa of the number are found to be new to Korea. Nitzschia longissima, Coscinodiscus occulus-iridis, Biddulphia sinensis, were the major species in the phytoplankton communities of the Bay. Leptocylindrus danicus, which was one of the dominant species in June, 1978, showed a distinct pattern of distribution;in June its abundance varied from 89.05% to 1.67% of total abundance along 4 stations. During summer months phytoplankton standing crops were ranged between 25,492 and 129,459 cells/l and the values of species diversity index (H) varied from 0.648 to 3.597. A brief taxonomic account is given.