Five biomarkers, catalase (CAT) activity, glutathione-S-transferase (GST) activity, the neural transmitter enzyme acetylcholinesterase (AChE), reduced glutathione (GSH) and malondialdehyde (MDA) levels were measured in specimens of Fulvia fragilis collected from Bizerta lagoon (Tunisia). Results demonstrated that F. fragilis showed differential biomarker response according to the importance of the anthropogenic pressure and the nature of pollutants that affect the lagoon. A clear organotropism was also observed with a higher biomarker response in digestive gland than in gills of this bivalve. These results indicate that F. fragilis constitutes a useful tool as sentinel organism for biomonitoring of aquatic pollution.