Dazeshan Reservoir of Shandong Province is a middle-sized one in which a complex fishfarming, i.e., fishculture with artificial fertilization integrated with net cage culture, was conducted. The seasonal variations of species composition, biomass, amount of chlorophyll and dominant species of plankton and the variations of those faetors in the summer and autumn of the fertilization cycles in the reservoir in 1988 wsre reported. 48 species of phytoplanlkton was found.Its yearly average biomass was 11.2mg/L. Cryptophyta dominated absolutely over the year. There were' five bio-mass peaks, one in the spring and the other four in the summer and autumn in 1988, of which the highest one appeared in the late summer. As for aooplankton, 28 species was found and its yearly average biomass was 0.74mg/L，and there were three biomass peaks in. the two same seasons mentioned above.Phytoplankton bio-mass reached a peak in each fertilisation cycle observed, but the time taken to reach the peak and the duration of the fertilization effect wars shorter in the summer one. Zooplankton biomass attained to a peak in the summer fertilization cycle,however, it was only fluctuating and rising gradually and did not appear a obvious peak in the autumn one.The affecting factors on the variation of plankton in the reservoir are also dis-cussed.