Abstract: On the basis of the data of four seasonal investigations in the East China Sea (23°30'~33°00'N,118°30'~128°00'E) from 1997 to 2000,the distribution and the seasonal changes of Ostracods abundance and the relationship between its abundance dynamics and environment and fishing ground were discussed in this paper through qualitative and quantitative methods.Results showed that the average abundance (2.57 ind/m³) of Ostracods peaked in autumn,which was equal to 41.59% of total abundance in four seasons.It was the second highest value (1.72 ind/m³) in summer,equaled to 36.55% of the total abundance.0.90 ind/m³ in winter and 0.70 ind/m³ in spring,which was the lowest value in four seasons.The dominant species of Ostracods separately were Euconchoecia chierchiae Muller in spring and winter,Euconchoecia maimai.Tseng in summer and Cypridina dentata (Muller) in autumn.Moreover,it was found that the total abundance showed linear correlation with surface salinity in spring and with surface water temperature in winter,while the abundance in summer and autumn did not show significant correlation with hydrogr aphic environmental factors.Ostracods,whose abundance was equal to 3.47% in the total zooplankton,only being lower. Copepods and Decapods in pelagic Crustacean,was an important pelagic animal.High abundance area of Ostr acods was located in north offshore of the East China Sea for four seasons while low abundance area occurred in nearshore of Zhoushan fishing ground.In south of the East China Sea,high abundance area of Ostracods was located in offshore in winter and spring but nearshore in summer and autumn.These suggested that Ostracods was a kind of typical warm water zooplankt on whose abundance was related to high water temperature seasons or warm water current.The agg regated behavior of Ostracods was dependent on the influence of its dominant species.It was also found that Ostracods,whose hig habundance area of very nearly overlapped the grazing areas of Todarodes pacificus,Navodon Septentrionalis and Scomber japonicus,became a kind of important diet for fish because of their characteristics of distribution and aggregation.