A phytoplankton study in the Korean waters has been commenced from 1964 as parts of the primary production studies in the Korean waters and the cruise for the cooperative studies of the Kuroshio from 1965 to 1968. Samples are taken by dipping 300-500 ml of sea water from the surface, and then fixed by adding neutralized formalin. The phytoplankton identification and cell counts are made on samples carefully after concentrated by decanting and centrifuging in the laboratory. This report deals with the surface phytoplankton obtained during the period of August 14-27, 1965 at twenty-three stations in the Korea Strait. Fifty nine species in seventeen genera of diatoms are detected from the samples collected at the stations. Among them seven species in five genera are considered to be purely neritic or cold water species and eight species in four genera are oceanic species of the Kuroshio. The highest phytoplankton standing crops are found in the sea area neighboring coastal waters of southern Korea, and the species distribution show anomalies in normally oceanic species being confined to neritic waters. There are distinct genera compositions of eastern, western and intermediate sea areas in the Kore Strait.