The localization of metals in selected tissues of metal-exposed mussels was investigated by means of autometallography. Mussels collected from a Zn-polluted site were (a) depurated or, alternatively, (b) exposed to either Cu, Zn or Cd for 41 d. Mussels collected from a clean site were used as experimental reference. Autometallographically demonstrated black silver deposits (BSD), indicating the presence of metals, were observed in gills, (a) in frontal cells and haemocytes of Cu-exposed mussels, (b) in secretory postlateral and abfrontal cells and in endothelial cells of Zn-exposed mussels, and (c) in frontal, postlateral and endothelial cells but mainly in abfrontal cells and haemocytes of Cd-exposed mussels. Autometallography also revealed the presence of BSD in connective tissue brown cells underlying the mantle. Additionally, adipogranular cells of the connective tissue surrounding the gonad follicles were positively stained but no BSD was found in gonad tissue. Scarce BSD were found in the cytoplasmic granules of the stomach wall of control mussels whilst depurating and metal-exposed mussels exhibited BSD lining the apex of the stomach epithelial cells. Contrary to the results previously obtained with marine gastropod molluscs, BSD were not found in the basal lamina of digestive tubules, stomach, and ducts of mussels. Highly conspicuous BSD were observed in digestive cell lysosomes of depurating and metal-exposed mussels. Basophilic cells were always devoid of BSD. Finally, BSD were also found in nephrocyte lysosomes and mineralized concretions.