The Mytilos project was carried during the last 3 years (2004-2006) in order to evaluate the level of chemical contamination along the coasts of the western basin of the Mediterranean Sea. Active mussel watch consisted of caged (12 weeks) Mytilus galloprovincialis at 123 stations along the Mediterranean coasts of Spain, France, Italy, North Tunisia, Algeria and Morocco. Results indicated total PAHs levels in the range 22-106 mu g/kg dry weight. All large towns or industrial areas from all countries of the basin were affected. Analysis of the origin of contamination demonstrated mainly a pyrolytic origin except in some areas such as Maddalena, Napoli, Alger, and Cornigliano (Genova). Statistical analysis of the PAHs accumulation data enabled the characterisation and ranking of each coastal area. The overall results give scientific and technical basis for large-scale monitoring of the PAH contamination in the entire western Mediterranean basin.