Abstract: Toxic tests of surfactant DBS(Dodecyl Benzene Sulfonate)were carried out at three biological organization levels: species, population. and community. Results showed that there were no specific differences in the acsute toxicity of DBS among five protozoan species: Paramecium caudatum, Tetrahymena americanis, T. piqmentosa, T. borealis 3IV WW0, T. borealis 3II VM665, and T. tropicalis DIV TC 89(P0.05). 12h-LC50 was between 7.0 and 10.47 ppm. At the population level of Tetrahymena americanis, the 12h-LC50 was 17.21 ppm; the effective concentration of DBS was 12.84 ppm(P0.05), based on the respiration of Tetrahymena americanis, determined by the Gilson Differential Respirometer. From low(3 ppm)to high(40 ppm)concentrations the toxic effects of DBS on the structure and function of the microbial community increased. The structural parameters included the number of protozoan speciesand their composition. The functional parameters included the colonization process(Seq, G, T90%)and the ratio of photosynthesis to respiration. There was a strong negative correlation between Seq and natural logarithm of DBS concentration; their relationship can be described by the regression equation Seq=29.7281—5.6722 InC.(P0.05). The effective concentrations(EC)were calculated from the equation, and the EC50, EC20, EC5 were 4.684, 0.51, 0.1681 ppm respectively. Based on 12h-LC50 at species and population levels, MATC was 0.70 and 1.721 ppm respectively when AF was 0.1, and 0.07, 0.1721 ppm respectively when AF was 0.01. Comparison between the MATC and the values of EC20 and EC5 reveals that the results from the microcosms are more scientifically-based and more realistic in the sense of environmental protection. Thus, the MATC in the Donghu Lake should be kept below 0.5 ppm or preferably below 0.17 ppm.