The fundamental objective of water treatment is the protection of consumers from pathogenic microorganisms. Chlorination of drinking water is essential to prevent waterborne disease. However, chlorine reacts with organic matter present in surface waters to form various by-products suspected of being carcinogenic. In the last decade, several epidemiological studies have been conducted to determine the connection between exposure to these chlorination by-products and human health defects. The purpose of this paper is to evaluate the genotoxicity of drinking water of Annaba city. The study have been carried out in different points of water distribution and in the station of treatment, using two tests of determination of genotoxic risk by means of SOS chromotest (using the strain Escherichia coli PQ37). SOS chromotest showed genotoxic effect of the sample collected from the exit of treatment station.