The water quality of Yongding Reservoir was evaluated by using aquatic ecological techniques combinedwith chemical and physical methods.Of the 14 biological,chemical and physical parameters examined inthis study,10,including primary productivity and chlorophyll a.were found to exceed the critical levelsfor eutrophic waters.Most of the indicators and Carlson’s Comprehensive Trophic State Index(TST)indi-ces indicated that the water quality of the western side of the reservoir was worse than that of the easternside.Polluted water from the Yongding River was considered a main source of pollutants in the reser-voir,while the farming of carp in net cages also contributed to the low water quality.A comparison ofthe water quality data in 1982 and 1988 indicated that the rate of eutrophication of Yongding Reservoir was alarming. The water quality of Yongding Reservoir was evaluated by using aquatic ecological techniques combined with chemical and physical methods. Of the 14 biological, chemical and physical parameters examined in th study, 10, including primary productivity and chlorophyll a. Were found to exceed the critical levels for eutrophic waters .Most of the indicators and Carlson’s Comprehensive Trophic State Index (TST) indi-ces indicated that the water quality of the western side of the reservoir was worse than that of the easternside .Polluted water from the Yongding River was considered a main source of pollutants in the reservoir, while the farming of carp in net cages also contributed to the low water quality. A comparison of the water quality data in 1982 and 1988 indicates that the rate of eutrophication of Yongding Reservoir was alarming.