Abstract: Ya-Er Lake is located in the west part of E-Cheng county, Hubei Province. In the past it covered about 6000 ha (now 2000 ha) and was one of the main fish production district in the province. Since 1962 the lake was polluted by wastewater of a chemical plant which manufactured parathion, malathion, dimethaote and BHC. The aquatic ecosystem was ruined. About forty percent of fish population were of abnormal form. After the ecological surveys carried out in 1972—1974, the pollution control of Ya-Er Lake was studied. The research works included: survey of the self-purification of pesticides of the lake, simulation experiment on the processes that would occur in the oxidation ponds, and the biological degradation of pesticides in aquatic environment, etc.The pollution control system of Ya-Er Lake consists of the following parts: 1. Oxidation ponds system. At one end of the lake, four oxidation ponds, connected in series, were constructed. The total area is about 187 ha and 3 m in depth. The retention time of wastewater is about 80 days when the amount of the effluent is 7×104 tons per day. The effluent from the pond system is then discharged into another pond for culturing fingerling fish. Therefore the nutritive elements contained in the effluent can be utilized by the fish and the eutrophication process is retarded in the receiving water body. 2. Sewer pipelines, 5.9 km long, between the chemical plant and oxidation ponds. 3. Two outfall canals (12.8 km long) which run along either side of the oxidation ponds collect and drain the rainwater into the lake. They are also used as water sources for farming. The above facilities were nearly completed in 1976. The operating experience for four years showed that the syatem was quite successful in district pollution control. The oxidation pond system can effectively reduce the organic wastes. The average removal efficiences for COD, organic phosphate, pnitrophenol, parathion, malathion, dimethaote and BHC are 77.3, 82.5, 99.3, 98.7, 98.4, 92.9 and 86.2 percent, respectivly. The daily waste-water load was 8×104 tons. Biological survey showed that the biotic community in the Ya-Er Lake ecosystem has been recovering since 1977. The rate of abnormal type of fish decreased to 0.1 percent in 1979. The BHC residues in fish decreased from 15—30 mg/kg to 0.55mg/kg (in skeletal muscle). The BHC residues of farm products in 1979 were 2.7mg—9.4 fold less than those before the construction of the oxidation ponds system.