Abstract: Six stations were set in cultural area of Daya Bay, the South China Sea, and water samples were collected every day from St.1 and St.2 and every three days from St.3-St.6 between April 7 and May 23 2000.Physical parameters (water temperature, salinity), chemical parameters such as dissolved nitrogen ( DIN, NO3- N+ NO2- N+ NH4- N), dissolved phosphorus (DIP, orthophosphates), dissolved silicate ( DSi), COD, meteorologic parameters, and biological parameters of chlorophyll a (Chl a), phytoplankton cell density and composition were measured.Population change of Chattonella antique /marina and its relationships with environmental factors and other groups of phytoplankton such as diatoms and dinoflagellates, were studied in this paper.The causative mechanism for the high occurrence of Chattonella was discussed as well.Generally, Chattonella has low capability in phytoplankton competition due to its low reproduce rate.However, Chattonella occurred in Daya Bay frequently in high densities in the spring of 2000, which the maximum cell density was 129 cells/mL.The appropriate water temperature, salinity and abundant nutrient contents in Daya Bay provided a suitable condition for the growth of Chattonella.On the other hand, low values of SiBN and continuous overcast and rainy weather in 2000 spring inhibited the growth of micro-diatoms, therefore, benefited the high propagation of Chattonella.Cyst has been regarded as the seed bed of motile cells in the water column of Chattonella and its blooms.However, it is necessary requirement for cyst to get through at least four months of compulsive resting stage under the temperatures below 11 e for maturation.The high winter water temperatures of Daya Bay can not provide the appropriate condition for Chattonella cysts to maturate.From the results, it could be suggested that high concentrations of motile cells of Chattonella in the spring of 2000, together with Chattonella blooms occurred in Daya Bay and other nearby sea areas, might result from the germination of mature cysts brought by the ballast waters.