This report describes the presence of a unique dual domain carbonic anhydrase (CA) in the giant clam,Tridacna gigas. CA plays an important role in the movement of inorganic carbon (Ci ) from the surrounding sea-water to the symbiotic algae that are found within the clam’s tissue. One of these isoforms is a glycoprotein which is significantly larger (70 kDa) than any previously reported from animals (generally between 28 and 52 kDa).