By means of a histochemical staining method, the sulphide-silver technique, the localization of cadmium was studied in several tissues of freshwater mussels,Anodonta cygnea andAnodonta anatina, exposed to CdCl2 (25 ?g/L Cd) for periods up to 16 weeks. After three weeks of exposure, reaction products indicative of the presence of free or loosely bound Cd were observed in the pallial mantle ... [Show full abstract] epithelium. After six weeks, reaction products appeared in gill epithelia of both species, and in kidney epithelia ofA. anatina. After nine weeks of exposure, reaction products were present also in kidney epithelia ofA. cygnea. In most cases, reaction products were concentrated in distinct particles in the apical cell region. The initial appearance of reaction products in mantle and gill epithelia was followed by a stationary phase, in which the reaction pattern did not change significantly, despite a continued accumulation of the metal. After 16 weeks of exposure, a further increase in the amount of reaction products was noticed in midgut gland and kidney epithelia ofA. cygnea, and in nearly all epithelia ofA. anatina. In the latter species, reaction products were no longer restricted to distinct particles, but were distributed throughout the epithelial cells and the underlying connective tissue.