Results from two ''Mussel Watch''-type monitoring programs were compared: the Réseau National d'Observation de la qualité du milieu marin (RNO), the French monitoring network, and the Mussel Watch Project of the U.S. National Status and Trends (NS&T) Program. 80 RNO sites (47 for mussels and 33 for oysters) and 89 NS&T sites (45 for mussels and 44 for oysters) provided a basis for the comparison of median concentrations of commonly measured trace metals (mercury, lead, zinc, cadmium and copper) and organic chemicals. Lower lead and lindane concentrations in the U.S. were explained by their respective history of use. Differences in Zn and Cu, essential elements for both mussels and oysters, could be due to specific internal regulation processes. Higher cadmium concentrations in the U.S. are possibly related to U.S. coastal areas being richer in nutrients or to a lesser use of this element as a general anticorrosive in France. We could not find any plausible explanation for higher mercury concentrations in France. This first attempt of a comparison of national chemical monitoring programs raises the need for deeper understanding of possible contamination sources.