Microalgae grown in ordinary air (0.04% CO2) show a much higher affinity for inorganic carbon (Ci) in photosynthesis than those grown with 2% to 5% CO2, although at saturating CO2 concentrations the maximum rate of photosynthesis is almost the same. This adaptation to a low concentration of environmental CO2 is explained by the so-called “CO2 concentrating mechanism (CCM)”, by which the CO2 concentration is raised at the reaction site of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco, the enzyme responsible for CO2 fixation in the Calvin = Benson cycle). CCM operates by a combination of several components such as energy-dependent Ci transporter(s), subcellular compartmentation, and carbonic anhydrase (CA; see Tab. 1), as reviewed by Aizawa and Miyachi (1984a), Tsuzuki and Miyachi (1989), Sultemeyer et al. (1993), Suzuki et al. (1994), and Badger and Price (1994). In algae, there is a variety of CA isozymes which differ in their structure, locality, and catalytic properties. In this chapter we review recent information on the diversity of algal and cyanobacterial CAs.