Abstract: The toxicity 24 h IC₅₀ of 18 chlorobenzenes to the bacteria in the Yangtze River was determined by using the bacteria growth inhibition test,and the range of-lg IC₅₀ was from 3.65 to 4.32,in which the least toxic compound was chlorobenzene and the most toxic compound was 1,2,4-trichlorobenzene.The quantitative structure-activity relationship（QSAR） models were developed for the toxicity by using the molecular connectivity index method.The toxicity of chlorobenzenes to the river bacteria is related to the kinds,amount and position of the substituents in benzene ring.The valence molecular connectivity index（）0ХV and（）⁷X_CH^V can well describe the toxicity of the compounds under study.The equation-lg IC₅₀=（0.839（））⁰X^V-25.372（）⁷X_CH^V+0.802 is stable enough to predict the toxicity, and the correlation relationship between the predicted and observed toxicity is up to 0.948.