The obseuations on the seasonal fluctuations of phytoplankton community in Haechang Bay of the Korean southern sea were carried out during four seasons from 1997 to 1998. A total of 77 species of phytoplankton including 10 freshwater species, belonging to 51 Benera was identified. Seasonal succession of dominant species was evident in Haechang Bay; Chaetoceros cunisetus, Skeletonema costatum, Eurampia zodiacus, Dictyocha fibula and Ceratium furca in summer, C. curisetus in autunm, C. cunisetus Rhizosoienia setigera and E. zodiacus in winter and S. costatum in spring. The phytoplankton community in Haechang Bay showed various species composition and was occupied with centric diatoms all the year round, Densities of the phytoplankton cell number by the samples of Haechang Bay ranged from $8.4{ imes}10^3;cells/l;to;2.0{ imes}10^5;cells/l$ with the mean value of $9.2{ imes}10^4;cells/l$ in summer, from $3.2{ imes}10^3;cells/l;to;4.6{ imes}10^6;cells/l$ with mean of $6,2{ imes}10^4;cells/l$ in autumn, from $8.4{ imes}10^3;cells/l;to;4.3{ imes}10^4;cells/l$ with mean $2.2{ imes}10^4;cells/l$ in winter and from $1.0{ imes};10^3;cells/l to;4.6{ imes}10^4;cells/l$, with mean of $1.1{ imes}10^4;cells/l$ in spring. Phytoplankton standing crops fluctuated with an annual mean of $4.7{ imes}10^4 cells/l$ between the lowest value of $1.0{ imes}10^3;cells/l$ in spring and the highest value of $4.6{ imes}10^5 cells/l$ in autumn, That is, phytoplankton standing crops was high in summer and autumn, while it was very low in winter and spring.