Carbonic anhydrase levels were examined in a variety of aquatic macrophytes from different habitats. In general, carbonic anhydrase levels increased across the habitat gradient such that activities were low in submersed aquatic macrophytes and high in emergent macrophytes with floatingleaved and free-floating plants exhibiting intermediate activities. Internal lacunar C0~ concentrations were analyzed in relation to carbonic anhydrase activities. There was no correlation between these two parameters. Internal C0» concentrations ranged from low to high in submersed macrophytes, but were low in floating-leaved and emergent macrophytes. The observed internal CO-concentrations are discussed in relation to the individual morphologies of the plants and the environments in which they occurred.