Carbonic anhydrase is a zinc metalloenzyme that catalyzes the simple interconversion between carbon dioxide (CO2) and bicarbonate (HCO3-). Seven genes encode the CA isozymes in vertebrates. They are single chain peptides termed CAI-VII. One CA isozyme is present in teleost fish. Three isozymes clearly appear together in birds. All seven types appear in mammals. Despite the great similarity among these isozymes, they present strong differences with respect to their kinetic properties. Many physiological and biochemical processes are related to the activity of CA isozymes.