The use of zebra mussel Dreissena polymorpha as a bioaccumulator for lipophilic compounds is nowadays standardized, but its employment in early warning systems by the biomarker approach is much less frequent. One of the main problems with the biomarker approach is due to natural variation of abiotic factors such as temperature that influence the activity of several enzymes. In this study, we investigated the influence of this environmental parameter on the activities of two different biomarkers: acetylcholinesterase (AChE) (inhibited by organophosphorus compounds) and CYP450 (inversely influenced by planar compounds and heavy metals). We used these two biomarkers to evaluate the environmental pollution of Lake Maggiore (northern Italy). Results showed a strong AChE inhibition in mussel specimens collected in some sampling sites of the lake, indicating heavy pollution by neurotoxic compounds. We also found a twofold effect on CYP450 activity, probably due to the activating effect of planar compounds and the inhibiting effect of trace metals.