Abstract: The present study is based on the analysis of the data collected in ten cruises during the periods of two surveys (from May 1992 to Jan.1993 and from Aug.1993 to May 1994).The results show that the number of phytoplankton species in highly eutrophic waters of Maluan Bay is less,only 66 species in total and 6~35 species for each cruise.The phytoplankton abundance is high.The mean cell density of phytoplankton from the net samples and the water samples reached 21 386×10⁴cell/m³ (from May 1992 to Jan.1993) to 110 136×10⁴cell/m³(from Aug.1993 to May 1994) and 1 150.7 cell/cm³ respectively.The mean concentration of chlorophyll a reached 6.67 mg/m³.Both the species diversity (H')and evenness (J) were low, on 0.78 and 0.28 respectively.The temporal and spatial fluctuation of the cell density of phytoplankton was quite significant,that was dominated mainly by the pennate diatom Nitzschia delicatissima which often occurried abundantly in closed waters riched in organic matter and could be taken as an ecologic indicator of highly eutrophic harbour.The existence of good correlation between the cell density of Nitzschia delicatissima and some environmental variables (e.g.PO₄^3--P and COD etc.)and the heterotrophic tendancy of the species are also discussed.