Decalcification at the mantle-shell interface in Mercenaria tnercenaria was studied through the changes in the chemical composition of the extrapallial fluid, and by the measurement of Ca45-deposition and solution. Measurements of O2-tension demonstrated that the clam was anaerobic soon after the valves were closed. Measurements of calcium, carbon dioxide, and hydrogen ion concentration showed that all of these components of the extrapallial fluid increase with increasing time of closure. These measurements, and measurements of calcium and succinic acid in the tissues and fluids of the clam, indicated thatsuccinic acid produced by the anaerobic metabolism of the clam was neutralized by the dissolution of previously deposited shell.