Abstract: Based on a fishery-biological survey of the Yao-Jiang River, ZheJiang Province, carried out during 1976-1977, the writer delineates the ecological properties of fish population and estimates the fish-producing capacity of this river.With the construction of dams across the River, the latter takes up some characteristics of a reservoir. The new environmental condition, especially the richness of plankton, is favorable to the development of freshwater fishery.There are 58 species of fish in the Yao-jiang River, with species composition similar to that of the lakes along the lower reaches of the Chang Jiang River. Owing to the practice of stocking, the fish community in the River is now a semi-artificial one. Population of bighead, silver carp, grass carp, black carp and blunt-snout bream are formed by artificial stocking; while common carp, crucian carp, Erythroculter dabryi, Acanthobrama simoni, Salangidae, Coilia ectencs, etc. represent the natural population. There is no serious interspecific competition, and these species are mainly supplementary in their interaction with reference to the utilization of spatial and temporal dimensions and food resources of the water body.The annual fish yield of the River from 1971 to 1976 averages 48.1 jin per mu, of which 65% are stocked species, 35% are natural population. And among the stocked species, bighead and silver carp constitute an overwhelming part. By making comparisons on the food resources, the growth rate and the population structure of fishes between the Yao-jiang River and the Qing-shan Reservoir in Hangzhou, Zhejiang Province as well as the Dong-hu Lake in Wuhan, Hubei Province, the writer points out that: (1) The plankton resources in the Yao-jiang River have not been fully utilized, and the stocking rate of silver carp should be higher than in the Tung-hu Lake and not less than that of the Qing-shan Reservoir, then the annual yield of silver carp and bighead put together would increase from the present 30.3 jin to over 60 jin per mu. (2) The food resources of grass carp, blunt-snout bream and black carp still remain to be brought into fuller utilization, but the potentiality of increasing the yield of these consumers seems rather limited. (3) The yield of natural fish stock such as the common carp, crucian carp, etc. has been more or less steady in recent years, and if due attention is paid to protective measures and to the natural proliferation of the stock, their production in the River may rise to a higher level. Accordingly, by rational stocking, by fuller utilization of food organisms as well as by protection of the natural fish stock, coupled with the improvements of fish screen installations, the yearly fish yield of the Yao-jiang River should in all likelihood reach the level of 80 jin per mu or so (around 600kg per hectare).