ABSTRACT: Intracellular storage and levels of mercury and selenium were studied in the livers of top marine predators belonging to different vertebrate taxa. Total mercury levels showed very important interspecific variations, ranging from 2.6 ug g^-1 intuna and swordfish to several thousand ug g^-1 (dry weight) in bottle-nosed and Risso's dolphins. However, methylmercury was less variable, ranging from 1 to 174 ug g^-1 (dry weight). The ratio between Hg and Se levels was close toequimolarity in marine mammals and cormorants, but a large excess of selenium in relation to mercury was observed in fish. Electron microscopy and x-ray microanalysis revealed mineral granules consisting of clustered crystalline particles in toothedcetaceans, sea lions and cormorants but not in tuna and swordfish. Granules containing mercury and selenium were mainly located in the cytoplasm of macrophages. These results suggest that the biosynthesis of mineral granules containing mercury andselenium in top marine predators is a common feature among these animals and that the existence of elimination pathways for the excretion of organic mercury might influence the amount of mercury and selenium stored as mineral granules in a particularspecies.