The spatio-temporal distribution and seasonal fluctuations of phytoplankton community were carried out in the Southwestern parts of Deukryang Bay of the Korean South Sea from July 1997 to January 1998. A total of 60 species of phytoplankton belonging to 41 genera was identified. In the southwestern parts of Deukryang Bay seasonal succession in dominant species; Eucampia zodiacus, and Chaetoceros spp. in summer, Nitzschia longissima, Chaetoceros curvisetus and Bacillaria paxillifera in autumn, Skeletonema costatum and B. paxillifera in winter, were very predominant. The community structure of phytoplankton in the southwestern parts of Deukryang Bay appeared to be diverse in species composition, and diatoms were most dominant through the year. Phytoplankton standing crops fluctuated with an annual mean of $1.2{ imes}10^5$ cells $L^{-1}$ between the lowest value of $8.0{ imes}10^3$ cells $L^{-1}$ in January and the highest value of $6.9{ imes}10^5$cells $L^{-1}$ by Nitzschia longissima in January. Densities of the phytoplankton cell number by the samples of the southwestern parts of Deukryang Bay ranged from $1.1{ imes}10^4$ cells $L^{-1}$ to $1.3{ imes}10^5$ cells $L^{-1}$ with the mean value of $4.1{ imes}10^4$ cells $L^{-1}$ in summer, from $1.0{ imes}10^4$ cells $L^{-1}$ to $6.9{ imes}10^5$ cells $L^{-1}$ with mean of $1.8{ imes}10^5$ cells $L^{-1}$ in autumn, from $8.0{ imes}10^3$ cells $L^{-1}$ to $4.6{ imes}10^5$ cells $L^{-1}$ with mean $1.6{ imes}10^5$ cells $L^{-1}$ in winter. That is to say, phytoplankton standing crops was high in low temperature seasons, while low in high temperature seasons. Chlorophyll a concentration fluctuated between 1.08 mg $m^{-3}$ and 21.6 mg $m^{-3}$ in January. In the southwestern parts of Deukryang Bay temporal change in chl-a concentration was not apparent. But chl-a concentration was high during a year. Therefore, phytoplankton production in the southwestern parts of Deukryang Bay could be very high year-round.