Kaozhouyang Bay is a typical semi2closed bay extended to the inland from Honghai Bay in South China Sea , which covers 29.7 km2 of total water area and 13. 73 km2 of beach area with the unique natural environment and predominant resource condition inside the bay. Being ranked to be the ecological mariculturing demonstration area in Guangdong Province , the bay was abundant in some special culturing species such as the famous“Tiechong Hao”(one species of oyster) . However , due to the lacking of planning program and scientific management , the phenomenon of disorderly development and over2utilization with the fishery multiculture in the waters of the bay in the recent years resulted in the frequent occurrences of fish diseases and then , to some extent , hampered the continual development of the fishery multiculture in Kaozhou Bay. In order to protect the ecological environment inside the bay , a survey on the ecological environment was carried out from 1999 to 2000. This paper reported the characteristics of the species composition and quantity distribution of phytoplankton in the bay. According to the result , altogether 188 species were identified in the survey waters , which belonged to 5 phyla with 142 species of Bacillariophyta , 22 species of Pyrrophyta , 5 species of Cyanophyta , 18 species of Chlorophyta and 1 species of Euglenophyta. Of all the phyla, diatom was the main predominant group. The species composition mainly consisted of the coastal species of fresh and salty2fresh , and the horizontal distribution and seasonal variation accorded with the water salinity variation in the survey waters. In the dry season , the predominant species changed complicatedly with an obvious regional succession characteristic and great variation , and the main predominant species were Rhizosolenia alata f . gracillima , Chaetoceros lorenzianus and Nitzschia paradoxa. While in the wet season , the predominant species was simple , and most of the waters was dominated by Skeletonema costatum and S . tropicum with the average predominant degree of 95. 8 % , and in the northwestern waters the predominant species changed to Merismopedia glauca and Skeletonema costatum. The seasonal variation and regional variation of the diversity index of phytoplankton were obvious and showed a trend that the dry season was higher than the wet season and the northwestern waters was higher than the southeastern waters. The mean cell density in the wet season reached to 26 471. 7 ?104 ind?m 3 , which was 66. 2 times that of the dry season. The distribution of the cell density showed the increasing trend from the northwestern waters to the southeastern waters.