Abstract: An investigation was made on the effects of chemical concentration, exposure time and developmental stage of the tadpole on the induction of mieronuclei in the erythrocytes of the forg tadpole (Rana nigromaculata) by two chemicals, ethyl methanesulphonate and Nmethyl-N'-nitro-N-nitrosoguanidine. Characters and correlations among several testing indices were analysed. Suggestions were made on the basic procedures and principles for a testing system, the micronucleus test on frog tadpole, for the detection of mutagens in aquatic environments. Besides, a unique cytological structure, the "Small body", was described, and the metabolic mechanism of micronuclei was discussed preliminarily. The testing system is sensitive, standardized and practical. It can be used not only for screening mutagens in water body and evaluating genotoxicity of chemicals, but also for detecting mutagenic activity of polluted fresh-waters.