The aim of this work was to investigate changes cytochromes P450 (CYP450) and some mixed – function oxidases (MFOs) activities in Mytilus galloprovincialis and Pernaperna exposed in situ to pollution in Agadir marine bay (South of Morocco). The main results showed a significant expression and induction of the analyzed activities in both mussels species. Thus, ethoxy-resorufin-O-deethylase (EROD) seems to be the most expressed MFO, while benzo[a]pyrene hydroxylase (BPH) would appear to be the most induced. 7- methoxy-resorufin-O-demethylase (MROD), benzyloxy-resorufin O-debenzylase (BROD) and pentoxy-resorufin O-depentylase (PROD) activities are also expressed to a minor degree; however, they tend to be induced by the pollution in the polluted site (Anza). The immunodetection of P4501A confirmed the results of the enzymatic measurements to which it seems to be complementary. These results provide a first baseline of mixed-function oxidases (MFO) in Mytilus galloprovincialis and Pernaperna living in the Agadir bay and enable us to propose the activities of these enzymes, in particular BPH and EROD, as a biomarker of environmental pollution.