Mean Cd, Cu and Pb concentrations in waters of the Celtic Sea surface layer were found to be 12.4, 55.8 and 36.6 ng.l-1 respectively. From 10 meters below the surface to a depth of 4,400 m, the values are of the same order of magnitude, even in oceanic waters close to the continental slope. Nevertheless, we noticed a wide spread from maxima to minima especially near the surface. On the basis of metal concentration data together with physical (temperature, salinity), chemical (phosphates, nitrites, nitrates, silicates, ammonium and oxygen) and biological (chlorophyll a and phaeophytin rates) parameters, the surface waters of the Celtic Sea can be subdivided into four zones: one inside the Ushant front; a second, called "mer d'Iroise", and finally, south and north Brittany waters. Plotted against depth, metal concentration data can be used to identify three water masses: the first at or near the bottom; the second at mid-depth, close to the thermocline; and the first at the surface, where high concentrations of Cu and Pb may be observed above the 200 m isbath.