In contrast to animal and plant carbonic anhydrases, relatively little is known about carbonic anhydrases in bacteria. Carbonic anhydrase activity has been well documented in a few bacterial species and its presence has been inferred on the basis of gene sequence homologies in several others, but their functions are generally not known. Two exceptions are the carbonic anhydrase (CA) that is a part of the cyn operon in Escherichia coli and the CA in the cyanobacterium Synechococcus PCC7942, both of which serve known specialized roles, i.e. prevention of CO2 depletion during cyanate degradation and concentration of CO2 for photosynthesis, respectively. The focus of this chapter will be on the properties and function of these two bacterial carbonic anhydrases. Since the function of the carbonic anhydrase in cyanobacteria is presumably closely related to the role of carbonic anhydrase in photosynthesis in higher plants, which is reviewed elsewhere in this book, more emphasis is placed on the carbonic anhydrase in E. coli. Although not eubacterial, a short section is included on the carbonic anhydrase from the Archaebacterium Methanosarcina thermophila. The distribution, properties, and possible functions and wider occurrence of carbonic anhydrases in other bacterial species are reviewed in the last section.