The general use of pesticides in various crops allows ensuring the quality of the crop production but has environmental impact, pesticide residues being often found in water resources, notably river in the agrosystems. As a result of repeated exposure to pesticides microorganisms can adapt to their degradation being responsible for enhanced degradation, an environmental friendly process diminishing the persistence of pesticide residues in the environment. This process can occur in the soil but also in the sediment of the river regularly exposed to pesticide contamination. To our best knowledge the ecotoxicological impact of multiple pesticides contamination on the degrading activity of microbial community of river sediment adapted to enhance biodegradation is not documented. In order to explore this question, we established an experiment using a sediment collected in the Morcille river (Beaujolais, France) known to harbor a microbial community adapted to the enhanced biodegradation of atrazine, 2,4-D and diuron. This sediment was either treated with glyphosate, or tebuconazole, or dimetomorphe or a mixture of these three pesticides or not treated (control) (four replicates per treatment). Samples were regularly collected to estimate the impact of pesticide contamination on (i) the ability of the microbial activity of the sediment to mineralize atrazine, 2,4-D and diuron by radiorespirometry and (ii) the abundance of atrazine, 2,4-D and diuron pesticidedegrading community was monitored by quantitative PCR targeting atzA/trzN, tfdA, and puhA/puhB genes using as template DNA extracted from the sediments. The results of this study will be presented and a first conclusion is that pesticide biodegradation potential of river sediment could constitute an interesting microbial functional bioindicator of pesticide effects. Acknowledgment: This work was funded by the APR Pesticide 2011 of the French Ministry of Environment within the framework of the Ecophyto program.