Symbiotic cnidarians absorb inorganic carbon from seawater to supply intracellular dinoflagellates with CO₂ for their photosynthesis. To determine the mechanism of inorganic carbon transport by animal cells, we used plasma membrane vesicles prepared from ectodermal cells isolated from tentacles of the sea anemone, Anemonia viridis. H¹⁴CO₃ uptake in the presence of an outward NaCl gradient or inward H⁺ gradient, showed no evidence for a Cl- or H⁺- driven HCO₃ transport. H¹⁴CO₃ and ³⁶Cl uptakes were stimulated by a positive inside-membrane diffusion potential, suggesting the presence of HCO₃ and Cl conductances. A carbonic anhydrase (CA) activity was measured on plasma membrane (4%) and in the cytoplasm of the ectodermal cells (96%) and was sensitive to acetazolamide (IC₅₀ = 20 nM) and ethoxyzolamide (IC₅₀ = 2.5 nM). A strong DIDS-sensitive H⁺-ATPase activity was observed (IC₅₀ = 14 µM). This activity was also highly sensitive to vanadate and allyl isothiocyanate, two inhibitors of P-type H⁺-ATPases. Present data suggest that HCO₃ absorption by ectodermal cells is carried out by H⁺ secretion by H⁺-ATPase, resulting in the formation of carbonic acid in the surrounding seawater, which is quickly dehydrated into CO₂ by a membrane-bound CA. CO₂ then diffuses passively into the cell where it is hydrated in HCO₃ by a cytosolic CA.