Specimens of the mussel Perna perna were collected along Moroccan coasts to determine the concentrations of polycyclic aromatic hydrocarbons ( PAHs) bioaccumulated in the tissues, and to measure benzo[ a] pyrene hydroxylase (BPH) activity. Chemical analysis of PAHs show that the Mediterranean (Nador, Martil, Tanger) and central Atlantic coasts ( from Rabat to Jorf Lihoudi) are those most contaminated ( 351 245 ng.g(-1) dry weight in Tanger). The mussel contaminants were of mixed origin for most of the locations with non negligible inputs of petrogenic origin in many of them. Baseline levels of PAHs were between 6 and 55 ng.g(-1) dry weight. BPH activity showed significant correlation (r(s) = 0.64, P < 0.05) with total PAH concentrations at the six most contaminated stations. The baseline level of BPH activity can be identified as in the range 0.1 to 13 pmol.min(-1). mg prot(-1) along the Mediterranean and Atlantic coasts.