Abstract: 120 liters tap water and head water from two water plants in S city were sampled in September 1998.XAD-2 resin was used to extract the organic matter in water.Gas chromatography/mass spectr ometer(GC/MS)was used to analyze the component of the organic extraction.In vitro Salmonella assay and in vivo mouse micronucleus assay were used to detect the genetoxicity of the organic extraction.The results showed some organic matters were detected in both tap water and headwater.The organic extraction had genetoxicity of different level and the genotoxicity of tap water is higher than headwater.The results of two different assays had accordance to some degree.It is indicated that the water of S city had been polluted by organic matter to some degree and chlorination can enhance the genotoxicity of water.The mechanism seems relevant to the by-product of chlorination.