During the process of maturity,a clear change occurred in using food resources with the development of fishes.There is usually an obvious niche shift in different ontogenetic stages,and the living environment and food groups would be converted with the increase of body size.Traditionally the method which is used for food shift is to analyze the contents in stomach and intestines,however,it could not completely show the location of fishes in food web as well as food source.In recent years stable isotopic way has been widely used to research food types of animals in different environments.The stable carbon isotope composition of consumers is able to show where the foods derive from,while the nitrogen isotope composition can show the position of trophic levels in food web.Mongolian culter(Erythroulter mongolicus mongolicus),which is one of the main economic fishes in Xiaojiang River after the ThreeGorges Reservoir accumulated water,plays an important role in aquatic ecosystem.The diet shift of Mongolian culter in various ontogenetic stages has a deep impact on the aquatic ecosystem.Therefore,based on the stable isotope analysis,combined with the identification of contents in stomach and intestines,we herein discussed the Mongolian culter food groups from Quma town and Huangshi town along the Xiaojiang River region so as to provide a theoretic basis for the management of Xiaojiang fishery resources and enhancement and releasing.Our results indicate that the δ¹³ C and δ¹⁵ N values of small Mongolian culter whose length is less than 200 mm are(-24.50‰±1.15‰) and (12.17‰±1.54‰) respectively,they are carnivorously omnivorous with 2.9 of trophic level; those big ones of more than 200 mm in length are(-23.87‰±1.12‰) and (13.54‰±1.12‰) respectively,their trophic level is 3.3 and diet type is carnivorous.The fact that the δ¹³ C and δ¹⁵ N values of large Mongolian culter are significantly higher than that of small ones(P<0.01), showing that a diet transfer took place during their growth,but there is an overlap of isotope values between large and small Mongolian culter for their sharing the foods from the same source.This study helps to predict the “topdown effect” caused by the enhancement and releasing of fishes and a variety of fisheries resources management activities such as fishery capture in Xiaojiang River region.