Mercury concentrations were measured in water, sediments, and three species of clams from lakes with and without reported mercury contamination. Elevated mercury levels in clams were associated with elevated mercury levels in water and sediments.Uncontaminated clams were exposed to three mercury compounds at 1.0, 10, 50, and 100 μg Hg/liter for up to 3 wk in the laboratory. The clams concentrated the metal in the order methylmercuric chloride > phenylmercuric acetate > mercuric chloride. The rate of uptake of mercury increased with increasing mercury concentration in the water. In most cases, temperature had no effect on the rate of mercury uptake or elimination.The distribution among organs depended on the compound to which the clams had been exposed. Only methylmercuric chloride was concentrated extensively in foot muscle. Transfer of methylmercuric chloride among organs apparently continued after exposure to the compound had ended.