The thylakoid membrane containing photosystem II (PSII membranes) from pea and wheat leaves catalyzed the reaction of CO2 hydration with low rate, which increased after their incubation either with Triton X-100, up to Triton/chlorophyll ratio 1:1, or 1 M CaCl2. The presence of the inhibitor of CAs, p-aminomethylbenzensulfonamide (mafenide), at the start line in the course of electrophoresis of PSII membranes solubilized by n-dodecyl-beta-maltoside (DM) decreased the amount of PSII core complex in the gel. The elution of PSII core complex from the column with immobilized mafenide occurred only either by mafenide or another inhibitor of CAs, ethoxyzolamide. The above results led to a conclusion that membrane-bound CA activity associated with PSII is situated in the core complex.