Based on the investigations from September to December in 2004 and during April in 2005, the diet composition of eight species of fishes in Lake Taihu was studied. The results were as follows: Coilia ectenes taihuensis Yen et Lin mainly fed on Cladocera accounting for 89.77±13.69% of individual composition in its food; Neosalanx tangkahkeii taihuensis Chen mainly fed on Copepoda accounting for 70.11%±10.57% of individual composition in its food; Microcystis occupied over 90% of volume composition of the food of Hypophthalmichthys molitrix (Cuvier et Valenciennes) and Aristichthys nobilos (Richardson); Water plant occupied over 70% of volume composition of the food of Cyprinus carpio Linnaeus; Microcystis accounted for 93.99±3.34% of volume composition of the food of Carassius auratus Linnaeus; Erythroculter ilishaeformis Bleeker and Erythroculte rmongolian Basilewsky mainly fed on Coilia ectenes taihuensis Yen et Lin and small-sized fishes. The conclusion was that the potential productivity of phytoplanktivores quickly increased, but the dominant fishes in fish catches were zooplanktivores fishes in Lake Taihu. For example, the potential productivity of phytoplanktivores was 10 234 t accounting for 37.0% of the total potential fish productivity in 2003. But phytoplanktivores catches were only 3637.7t accounting for 10.9% of the total fish catches in 2004, while zooplanktivores catches accounted for 80%-90% of the total fish catches. Therefore the present composition of fish catches was not consistent with the food resource in Lake Taihu. Additionally, the effects of the increases of zooplanktivores fishes maybe indirectly probably enhanced the development of Cyanobacteria by feeding on large amount of zooplanktons. The effects of Carassius auratus Linnaeus on environment should be revaluated because the fish mainly fed on Microcystis.