Abstract: This paper reports the changes of community structure of the benthos and their succession after the oxidation ponds had been built on Yan Jia Lake, and three monitoring methods.After survey of two years, a total of 37 species of benthos was found, including 12 species of oligochaetes, 8 species of molluscs, 15 species of aquatic insects and 2 species of others. The three methods are: (1) Using the changes of the community structure of the benthos. According to the proportion of the oligochaetes, aquatic insects and molluscs occurring in the water contaminated by pesticides to various degrees, waters may be divided into six grades-extremely heavy pollution, heavy pollution, moderate pollution, light pollution, slight pollution, and clear water. (2) Using the diversity index of benthos community. The result obtained from calculation with the formula of Margalef and Shannon Weaver was compared with the analysis of chemicals, and it is suggested that the pollution of pesticides be divided into six numerical degrees, namely, zero showing extremely heavy pollution; above zero but less than one showing heavy pollution; between one and two showing moderate pollution; two to three point five showing light pollution; above three point five to five showing slight pollution; and a,bove. five showing clear water. (3) Analysis of the residual accumulation in benthos. The residue of BHC in Limnodrilus hoffmeisteri, Glyptotendipes sp., Bellamya acruginosa and Anodonta pacifica was determined. The author considers that Anodonta is a favorable material for monitoring the accumulation of pesticides.These three methods were employed in 1972—73, 1976, and 1977 with satisfactory results, so the author considers that these methods are effective for monitoring the waters polluted by pesticides. Among them, the method of community structure of benthos seems to be the superior one.The community structure of benthos has changed a lot since the completion of the oxidation pond system above the Yan Jia Lake, such as the oligochaetas being decreased to 50%, while the aquatic insects being increased by 12 times, the mollusca being increased by 3 times. These changes are due to the purification capacity of the oxidized pond system.