Measurement of carbonic anhydrase (CA) activity in homogenates of accessory boring organs of the muricid gastropod, Purpura lapillus , by Meldrum and Roughton's manometric method showed that CA is always present in boring as well as in inactive ABOs, but in variable amounts. Tests by the same method on whole ABOs in an isotonic solution were negative, proving that CA remains intracellularly. Experiments in vivo on inhibition and activation demonstrated clearly that CA is responsible for demineralization of the valves of lamellibranchs by Purpura: (a) at low concentrations of Diamox, partial inhibition of the enzymeoccurred, that is, the number of complete holes decreased or disappeared and etchings increased; and at higher concentrations full inhibition took place. Inhibition is reversed when snails are replaced in normal sea water; (b) action of pure CO2 or mixtures of CO2 and O2 accelerated boring: in the optimal mixture, three times more boreholes were produced by snails than in the controls, and in about half the time. Under these conditions the reaction catalyzed by CA goes to the right with hydration of CO2 and is accompanied by release of H+ ions; in the presence of an increased concentration of CO2, the reaction is intensified and results in an additional release of H+ ions. Consequently, destruction of CaCO3 by the ABO of Purpura in sea water enriched with CO2 is accelerated. To identify the nature of the exchanged ion, we tested the effect of NaCl and KC1 on the boring mechanism in vivo by increasing the amount of these salts in the seawater of snails with prey. In this case augmentation of the boring activity was noticed. These results suggest that the boring activity is accompanied by complex ionic exchanges.