The influence of silver carp (Hypophthalmichthys molitrix) and bighead (Aristichthys nobilis) on the zooplankton community structure in Lake Donghu, Wuhan was studied using enclosure method during May-October 1989 and April-September 1990. Species composition, standing crop and body size of zooplankton showed remarkable differences between enclosures with and without fish. In the enclosures in which silver carp and bighead were stocked, the average densities of the Protozoa and some small-sized rotifers were high, while those of the large-sized zooplankton were low. The crustacean percentage was on average less than 2% of the total number. The blue-green algal "water bloom" did not appear in 1989-1990. In the enclosures without fish, the average densities of the Protozoa and some small-sized rotifers were low, while those of the large-sized some small-sized rotifers were low, while those of the large-sized zooplankton were high. Especially, the large-sized Daphnia was dominant during April-June 1990. The crustancean percentage was on average more than 8% of the total number (1990). A heavy "water bloom" of the blue-green algae appeared during June-September in 1989-1990. The average biomass of zooplankton was much higher in the two enclosures without fish (55.035 mg/l 38.374 mg/l) than in the two enclosures with fish (3.028 mg/l and 3.490 mg/l) during April-July 1990. The examination of the size-frequency distributions of the crustancean (copepod nauplius excluded) showed that the frequency of the large sized crustacean was higher in the enclosures without fish that in the enclosures with fish. For example, on May 7, 1990, the frequency of crustacean (>1.5mm) in the enclosures without fish was about 13 times as high as in the enclosures with fish. The influences of silver carp and bighead on the zooplankton were different. The standing crop of Protozoa and Rotifer was lower in the enclosure with silver carp than in the enclosure with bighead, while the standing crop of the crustacean showed a reverse result. The enclosure experimental results showed that the feeding of silver carp and bighead had resulted in the dominance of small-sized zooplankton instead of large sized ones.