In order to clarify water quality, dynamics and structure of phytoplankton communities, and red tides, the present study was carried out monthly from July to September 1998 at 19 stations in Deukryang Bay. Water temperature varied from 24.$0^{circ}C$ to 28.6$^{circ}C$, and salinities ranged from 25.0$ extperthousand$ to 28.6$ extperthousand$. During red tides in July, chlorophyll-$alpha$ contents were rather high in August, it showed that phytoplankton controlled primary production in this bay. Phytoplankton was composed of 89 diatoms, 19 dinoflagellates, and 3 silicoflagellates. Phytoplankton standing crops varied from a minimum of 1.3$ imes$10$^4$cells/ι(Sept., St. 17) to a maximum of 3.8$ imes$10$^{6}$ cells/ι (July, St. 10). Red tides occurred in July when the standing crops averaged 1.8$ imes$10$^{6}$ cells/ι. Leading the causative organisms of red tide were Prorocentrun minimum in the upper bay, Chaetoceros curvisetus in the mouth and middle part of the bay, Ceratium furca and Thazassio- sira sp. around Nokdong Harbour. Concentration of nutrients such as nitrogen was high in the upper bay, during red tides, water temperatures varied from 23.8$^{circ}C$ to 29.7$^{circ}C$ and salinity were 23.l$ extperthousand$ to 27.0$ extperthousand$.