The distribution networks of water seldom function in stationary conditions. According to many parameters (quality of water of the resource, chlorine rate applied, technical interventions...), networks are constantly subjected by hydraulic, biological as physicochemical discontinuities. This study concerned disinfectant discontinuities (bleach). Few work already published on this topic thus identified a risk of destabilization of the water quality in the supply network under these discontinuities, but this work is limited to discontinuities "all-or-nothing". To our knowledge, no data published exists on discontinuities with a "low and high" sequence of chlorine dosage (as with the “Vigipirate plan”). The objective of this study was thus to appreciate the impact of change of a chlorine concentration (chlorine sequence: 0.1 - 0.4 - 0.1 mg Cl2/L) on the biofilms and the water quality in drinking water distribution system. Methodology The assays were performed on an experimental DS composed of a loop (cast iron cement-coated) supplied with drinking water of Nancy, which was re-chlorinated on site in order to obtain residual of chlorine in the distributed of 0.1 or 0.4 mg/L. The network is equipped with coupons allowing the analysis of the biofilm samples. Discontinuities were applied following the sequence: 0.1 - 0.4 - 0.1 mgCl2/L. During these chlorination periods, the quality of water and biofilm was characterized in term of total number of bacteria, culturable bacteria, bacteria with a membrane integrity (kit BacLight). In addition, the phylogenetic composition was evaluated using FISH method (Fluorescent in situ hybridization), by targeting the populations of Eubacteria (probe EUB338), and of alpha-, beta-, gamma-proteobacteria (probes ALF1, BET42a and GAM42a) (Amann et al. 1990). Results  Effect of chlorine discontinuities on the bacteria present in water Chlorine discontinuities (from 0.1 to 0.4 or from 0.4 to 0.1 mgCl2/L) did not involve any modifications of the microbiological quality of water, nor of its phylogenetic composition. Only the bacterial culturability was modified as a function of chlorine concentrations: a reduction when chlorine dosage increased and reciprocally.  Effect of chlorine discontinuities on the bacteria present in the biofilm The phylogenetic composition of the biofilm exposed to chlorine discontinuities was modified and adjusted according to the oxidizing conditions applied. A chlorinated biofilm contained on average 106 cells/cm2 of which less than 10% are culturable, and around 30% are detected by FISH. Among this Eubacteria population, the alpha-proteobacteria was in the majority (37%), the beta-proteobacteria accounted for 1.6% and the gamma-proteobacteria less than 0.1%. When chlorine dosage was increased from 0.1 to 0.4 mgCl2/L, an significant increase in beta- and gamma-proteobacteria (which remain however minority :< 5%) and a reduction in the alpha-proteobacteria occurred. Conversely, when chlorine concentrations were again decreased to 0.1 mgCl2/L, a significant increase in the alpha-proteobacteria of the biofilm occurred, whereas the gamma-proteobacteria decreased.