Abstract: By means of biological monitoring method,using international standard indicator organism,scenedesnus obliquus,the acute toxicity of many organic acids are studied.The varying degrees of toxic acids with the relation to their molecular structure have also been discussed.The results showed that the toxicity of aromatic acid,expressed in median effective concentration (EC₅₀) in 96 h,is much stronger than the fatty acid and the toxicity of some tested derivatives is much stronger than that of mother substances.The toxicity is slightly weak for the acids with short carbon chains and for the acids soluble in the water.