Many surface waters have been reported to be contaminated with genotoxic substances whereas the most critical sources remain to be identified. The wastewater treatment plant (WWTP) effluents discharged in surface waters is one of the significant sources. Indeed, these effluents often contain mutagens although wastewaters are treated through different processes (Ohe, 2004). Invertebrates constitute the major part of the aquatic biomass, play an essential role in ecosystems and are currently used as sentinel species in ecotoxicological studies. However, there is an obvious lack of genotoxicity biomarkers in aquatic species such as crustaceans. Recently, genotoxicity assessment has been carried out in an ecologically relevant gammarid species, Gammarus fossarum using the comet assay (see MO 182 poster). The aim of this study was to assess genotoxic stress of WWTP effluents on caged gammarids in the Ardière River, Bourbre River and Saône River.