The bivalve mollusks are among the aquatic bioindicators that are commonly used in monitoring water pollution studies, thanks to their behavior and metabolism. They are directly affected by the level of pollution in a given site. During this research, the study of the biological response in gills, hepatopancreas and muscles of indigenous mussels Mytilus galloprovincialis were used for monitoring, emissary's pollution in four polluted sites in the coastal environment of Casablanca. Seasonal variations of the activity of antioxidant defence enzymes, catalase (CAT), glutathione S-transferase (GST), as well as lipid peroxidation (LP) were measured as biomarkers within a one year period and compared to mussels from an unpolluted sampling site. This study was completed by analysing a series of abiotic factors (temperature, pH and conductivity) and chemicals (heavy metals; Hg, Pb, Cu) into seawater. Our result showed that the availability of metallic contamination and the environmental stress conditions causes relatively an oxidative stress in this species at each station studied. While the pollution's level clearly varies according to the sampling campaign. Furthermore, they revealed a significant increase in GST activities and LP concentrations and significant decrease in CAT activities in mussels collected in sites with industrial contamination. This negative correlation suggested that the organisms at this location are exposed to a relatively higher level of oxidative stress. This first study in the area confirm that variations of antioxidant defence enzymes activities and LP concentrations in mussels could be used as prospective biomarkers of toxicity in environmental monitoring programs. (C) 2016 Elsevier Ltd. All rights reserved.