Inorganic carbon acquisition under emerged conditions was studied in the red macroalga Bostrychia scorpioides by investigating the effects of changes in the pH of the water film surrounding the thalli when emerged and of inhibitors of carbonic anhydrase (CA; EC 4.2.1.1). The CO2 uptake rate increased by approximately 150% when the pH of the water film was changed from 8·1 to 10·5. Acetazolamide (AZ), an inhibitor of the external CA, inhibited CO2 uptake by 50% at pH values higher than 8·7. The effect of ethoxyzolamide (EZ), an inhibitor of both external and internal CA, was more pronounced than that of AZ, inhibiting the CO2 uptake rates by 80% at all the pH values assayed. At pH 10·5, CO2 flux into the cells exceeded by several orders of magnitude the flux supported by the spontaneous dehydration of HCO3− to CO2 in the water film. From these results it was concluded that the main source of inorganic carbon was atmospheric CO2. The driving force for the flux of atmospheric CO2 into the cell must be the CO2 concentration gradient. Therefore, low internal CO2 concentration might be necessary to account for the CO2 fluxes measured. The function of the internal CA could be to speed up the transformation of CO2 into HCO3− inside the cell. The function of the external CA could not be fully described from the experiments although it was demonstrated that the external enzyme enhanced the CO2 flux from air to cell surface.