The toxic effects of LAS (Linear Alkyl Benzene Sulfonate), PCP-Na (Soldium Pentachlorophenate) and Potassium bichromatum on protozoan: Mainly Tetrahymena pyriformls, were studied at four biological organization levels: subcell. cell, population and mesocosm. (1) The effluents purlfying efficiencies were firstly eveluated by using microbial communities. In the middle-sized simulated tests of ecological engineering system for effluents purification-resourcement in Changde, Hunan province. Anionic surfactants in effluents were purified gradualiy through a series of oxidation ponds. Finally, the concentration of surfactant was decreased to the standard of driakingwater quality at the fourth pond. According to the structural and functional parameters of microbial communities, such as the number of protozoan species the sepecies composition, heterotrophic index, diversity index and the protozoan colonization (Seg, G, T_(90%)), the toxic stress decreased in sequence as precipitation tank, the second pond, the third pond and the fourth pond. (2) Acute and chroaic toxicity tests at the population level of tetrahymena pyriformis SI showed that 12hr-LC_(50) of PCP-Na was 0.117ppm, 12hr-LC_(50) of K_2Cr_2O_7 was 14.098ppm and 24hr-IC_(50) of K_2Cr_2O_7 was 3.23ppm. (3) Toxicity tests at the cellular level DNA amounts in macro-and micronuclel of Tetrahymena pyriformis SI treated with LAS, PCP-Na and K_2Cr_2O_7 were measured by Feulgen microspectrophotmetry. Micronuciel (MN) of Tetrahymena pyrlformis BJ_4 were also identified n the meantime. It was found that 0.14ppm (bleow MATC) conid damage genetical materials of the cells. The safe concentration of LAS should be 0.014ppm. 0.05ppm K_2Cr_2O_7 potassium blchromatum (equal to MATC) and 0.0009ppm PCP-Na (below MATC) had potential harmal effects at the genetical level. (4) Toxicity tests at the subcelluiar level TEM and SEM techniques were applied to observe the ultrastructural changes of treated cells. The most sensitive organelle of damage of LAS was the mitchondrion. And it would apper that the site of damage of K_2Cr_2O_7 was nucleus. Spetrophotometric techaluqes were used to measure enzymes activities of the cells. And the results showed that 11.31ppm LAS, as well as 0.14ppm LAS had altered SDH and IDH activities. The toxic mechanisms of LAS to Tetrahymena pyriformis were also discussed in this paper.