Abstract: The toxicities of single and binary mixtures of aniline and nitroanilines to Daphnia magna were assayed respectively.Additive Index( AI ) and Similarity Parameter( λ )were used to evaluate the joint toxicity.The two methods yielded one consistent conclusion,i.e.all the joint actions of the three binary mixtures were synergistic ones.Compared the AI and λ values when the binary mixtures were combined at equal quantity and the toxic unit ratio is respectively 1∶1,1∶4,2∶3,3∶2 and 4∶1,the results indicated that the mixture combined at equal toxic unit had the strongest synergistic action.