Abstract: This paper embodies the results of studies on the composition and seasonal abundance of planktonic copepods from February 1984 to February 1985, in Lake Dian-Shan, Shanghai.Samples were collected monthly at three sampling stations. Eleven species of planktonic copepods were found in the lake. According to their seasonal occurrence, it is possible to divide them into four types: yearround type, summer-autumn type, winter-spring type, and evading severewinter type.In the pattern of seasonal fluctuation, the peaks of copepod abundance were usually recorded in spring, summer and autumn, with the third peak far exceeding the formers and almost persisting for two months. The changes of copepod abundance were mainly determined by the populations of four dominant species: Limnoithona sinensis, Sinocalanus dorrii, Mesocyclops leuckarti and Cyclops vicinus vicinus.There were four peaks of the biomass of planktonic copepods in a year. The peaks of biomass in spring and autumn occurred after the peaks of abundance in corresponding seasons because 80—90 percent individuals were larve at the time of maximum abundance.Ecological factors that effect the seasonal abundance and biomass of the copepodes are discussed briefly.