Phytoplankton samples for this study were collected by Van Dorn Sampler at 28 oceanographic stations in the southern waters of Korea in February, April and August, 1988 to characterize the distribution and species composition of phytoplankton and their relation to the movement of water masses. During the study periods, phytoplankton standing crops in the southern waters were maximum in April and minimum in February. When the geographical distribution and abundance of the phytoplankton were considered, the highest standing crops were found near the front zone formed between Cheju Island and Tsushima Island. Therefore, their abundance depends ell the location of the front zone. On the other hand, there were relatively low standing crops in the open sea off the coastal area from the front zone in February and August. The cause of the unusal low standing crops found in August at the station 205/01, the central part of the coastal area of the southern waters where the largest standing crops occurred in February and April, seems to be due to the intrusion of Tsushima warm waters. This is because the waters at the station have high temperature and salinity, poor dissolved oxygen and nutrient salts, which are identical to the characteristics of proper Tsushima warm waters. It seems that these warm waters may affect the poor productivity in this area. Seventy-two species in thirty-three genera of the phytoplankton were indentified from the samples. Among them, 61 species in 27 genera belong to diatoms and 12 species in 7 genera to dinoflagellates. The largest number of phytoplankton species occurred in August while the largest number of the tells in April. Predominant species were Eucampia zodiacus in February, Skeletonema costatum and Chaetoceros curvisetus in April and Chaetoceros affinis in August. Eucampia zodiacus which was dominant in February and April seems to be a indicator of the southern coastal waters of Korea and mixed waters west of Cheju Island.