The measurement of inorganic carbon dependent photosynthetic oxygen evolution in air-grown cells of Phaeodactylum tricornulum Bohlin has shown that sodium ions increased the affinity of the cells for bicarbonate. Lithium ions (50 mmol 1−1) inhibited bicarbonate-dependent photosynthetic oxygen evolution but this inhibition was overcome at saturating bicarbonate concentrations. Bicarbonate-dependent photosynthetic oxygen evolution was also inhibited by lithium ions in cells of Phaeodactylum grown in high CO2, (5 % v/v). Acetazolamide and ethoxyzolamide were potent inhibitors of carbonic anhydrase in cell extracts. At pH 8.0 the K0.5(the concentration of inorganic carbon required for 50% or the maximal rate of photosynthetic oxygen evolution) was increased from 53 to 542 μmol I−1 in the presence of ethoxyzolamide whereas in the presence of acetazolamide the K0.5.(CO2) increased to 130 μmol 1−1. In the presence of ethoxyzolamide the internal dissolved inorganic carbon (DIC) concentration was increased but this increase was prevented in the presence of lithium ions when bicarbonate transport into the cell is inhibited. These results are in agreement with bicarbonate transport across the plasmalemma and intracellular carbonic anhydrase increasing the steady-state flux of CO2 from inside the plasmalemma to ribulose-1,5-bisphosphate carboxylase-oxygenase by facilitating the interconversion of HCO−3 and CO2.