The role of carbonic anhydrase in the carbon-concentrating-mechanism of bryophytes of the class Anthocerotae was investigated by comparing the gas-exchange characteristics of material which had been incubated in the membrane-permeable Carbonic Anhydrase inhibitor ethoxyzolamide, with those of untreated material and material which had been incubated in buffer solution. In Phaeoceros laevis (Anthocerotae), incubation in ethoxyzolamide caused a depression in the rate of gross assimilation and a decrease in CO2 affinity beyond that which could be attributed to increased diffusion limitation. A range of liverworts and mosses, in which a carbon-concentrating-mechanism is absent, were also investigated. These showed no depression of rates of gross assimilation after incubation in ethoxyzolamide relative to those of untreated material. The CO2 compensation point and CO2 uptake characteristics of Phaeoceros laevis were significantly affected by incubation in ethoxyzolamide. Values of CO2 compensation point for Phaeoceros laevis rose from 2.5 Pa, after incubation in buffer, to 20 Pa after incubation in ethoxyzolamide. The CO2 compensation point for the liverworts Pellia epiphylla and Marchantia polymorpha was not significantly affected by incubation in ethoxyzolamide. Measurements of the release of CO2 at the end of a short (15 min) period of illumination revealed that, after suppression of carbonic anhydrase activity, the rapid release of a CO2 pool occurred in Phaeoceros laevis but not in the liverworts. There were also significant differences between values for fractionation measured in units per mil (‰), measured instantaneously, for Phaeoceros laevis incubated in ethoxyzolamide, compared with fractionation values for this species after incubation in buffer. Incubation in ethoxyzolamide caused fractionation values to rise from 12.4–22.7‰, indicating that the carbon-concentrating-mechanism of this species had been inactivated. Incubation in ethoxyzolamide had no effect on fractionation values for the liverworts. The convexity of the light saturation curves of liverworts and Phaeoceros laevis was also investigated, but there were no differences between groups before or after the two treatments. The data indicate an important role for carbonic anhydrase in the functioning of the carbon-concentrating-mechanism in the Anthocerotae.