Abstract: In this study, a micronucleus test using the erythrocytes of the tadpoles of the frog (Rana nigromaculata Hallowell) was developed as a new monitoring system for the mutagenic acti vity of the city sewage. In an experiment in which the tadpoles were exposed to domestic sewage for 16 days, the frequency of micronucleated erythrocytes showed a significant elevation after 2 days of exposure, and then increased over time to a maximum after 12 days. In another experiment in which the tadpoles were exposed to mixed sewage of different concentrations, the frequency of micronucleated erythrocytes showed significant dose-dependent increases. The micronucleus test was used to evaluate the efficiency of the Synthetic Biological Ponds System in Huangzhou city for reducing the mutagenic activity of city sewage. As the mixed city sewage flew through the biological ponds, the frequency of micronucleated erythrocytes induced by the sewage decreased gradually from 7.54％ in the influent to 1.52％ in the final effluent, approaching the level in the control (1.07‰). The results indicated that both domestic and mixed sewage have strong mutagenic activity. The Synthetic Biological Ponds System is an efficient method to reduce the mutagenic activity of city sewage. The micronucleus test using erythrocytes of frog tadpoles is a sensitive,practical and easy-to-apply monitoring method.