Coilia ectenes taihuensis Yen et Lin(C. ectenes) is the dominant fish species in Lake Taihu, and the catch has been increasing with the eutrophication of Lake Taihu. In this study, in order to investigate the seasonal diets variations of C. ectenes in Gonghu Bay of Lake Taihu, we sampled the fish by gill net and analyzed bimonthly the gut contents of C. ectenes from April to December, 2009. Meanwhile, selectivity for different food items was also analyzed for the purpose to study the food preference of the fish. The results showed that the food composition of C. ectenes varied from month to month. Zooplankton, fish larvae, shrimp and insects were found in the stomach contents of the fish, but zooplankton was the dominant food. Among the zooplankton species, Copepod was the main food in April with close mean percentages diet composition of Copepodite, Sinocalanus dorrii Brehm and Cyclops spp. In June, August and October, C. ectenes fed mainly on cladoceran with Moina spp. as the dominated mean percentage diet composition in June and Bosmina spp. in August and October, respectively. Small zooplankton composed the main percentage diet composition, such as Bosmina spp. and Moina spp., in all sampling months. However, C. ectenes negatively selected Bosmina spp. for their food, but positively selected Moina spp. and other large-sized zooplankton species. But when Daphnia spp. and Leptodora kindti Focke were present in the sampling months, C. ectenes showed high selectivity for them. From this investigation, we can conclude that C. ectenes feed mainly on planktonic crustaceans, and showed high selectivity for large-sized zooplankton. Moreover, fish larvae, shrimp and insects may be their potential food for the fish when its total length is longer than 130 mm. Different food items varied largely in fish food composition among different seasons. Studying on the diet composition of C. ectenes can help us understand the seasonal effects of C. ectenes on zooplankton community structures better, and provide evidences and foundations for making rational plans of fisheries and lake management.