Abstract: The phytoplankton-silver carp subsystem in the Guozhenghu basin ecosystem of Donghu(East Lake) was examined for the purpose of a quantitative evaluation of the interaction between the two.The growth rate of silver carp during the most productive 60 days in the lake and that of the sufficiently fed fish were compared. The result indicates that no difference existed between the rates in the first 20 days, while in the later 40 days the rate of the lake fish was lower than that of the sufficiently fed fish.The result of application of the simulation technique to the interaction reveals that such a retardation in growth of the lake fish was due to lowering of the biomass of phytoplankton caused by fish feeding and consequently to insufficient ingestion by the fish.The nitrogen budget of the subsystem indicates that the phytoplankton mass produced in the season was entirely consumed by the carp and that the carp supplied 78% of the total nitrogen source which was required by phytoplankton. The result suggests that the interacting subsystem is quantitatively the most important one in the lake.