HCO3(-)-stimulated ATPase activity was demonstrated in mantle tissue of the freshwater clam, Anodonta cataracta. 2. Calcium (1 mM) slightly inhibited and SCN- completely inhibited HCO3(-)-stimulation of the enzyme. 3. ATPase activity had a Km of 6.8 mM for HCO3(-)-activation and was inhibited at HCO3(-)-concentrations greater than 20 mM. 4. Subcellular fractionation studies revealed the presence of both a mitochondrial and a non-mitochondrial HCO3(-)-ATPase.