The toxicity and the chemical quality of three municipal wastewater effluents have been studied. Acute and sub-chronic bioassays were carried out. We also measured the induction of the hepatic ethoxy resorufin-O-deethylase enzymatic activity (EROD) of fish exposed to the effluents. Chemical analysis allowed to identify the most frequent substances detected in the effluents. From chemical data and results of bioassays we conclude that ammonia concentration explains a part of the lethal toxicity. But chronic toxicity tests carried out with whole effluents or extracted chemical fractions showed that organic pollutants detected in the effluents could also cause long-term effect. All the tested effluents induced fish EROD activity. The instream invertebrates monitoring confirmed the municipal wastewater treatment plant impact on the receiving water.