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We have assessed the effect of vanadate as an inhibitor of plasma-membrane ATPase on photosynthesis and the ATP/ADP ratio in Chlamydomonas reinhardtii CW-92 (a mutant strain lacking a cell wall). This effect was compared in low-CO2-adapted cells grown in media bubbled with air containing 400 or 70 µL · L-1 CO2. Evidence is presented indicating that cells grown at 70 µL · L-1 CO2 have a higher rate of photosynthetic O2 evolution than cells grown at 400 µL · L-1 CO2, at limiting carbon concentrations. Extracellular and intracellular carbonic-anhydrase activities were, however, similar in cells grown in both of the low-carbon conditions. Vanadate inhibited, to a different extent, the HCO3--dependent O2 evolution in cells grown at 400 and 70 µL · L-1 CO2. At 400 µM vanadate, inhibition reached 70–75% in cells grown at 400 µL · L-1 but only 50% in those grown at 70 µL · L-1 CO2. The ATP/ADP ratios determined with and without vanadate at limiting concentrations of dissolved inorganic carbon indicated that more ATP was hydrolysed in algae grown at 70 µL · L-1 than in those grown at 400 µL · L-1 CO2. We conclude that the maximal capacity to accumulate dissolved inorganic carbon is inversely related to the CO2 concentration in the medium. Activation and — or synthesis of vanadate-sensitive ATPase may be the major explanation for the higher capacity for HCO3--dependent O2 evolution in cells grown under limited CO2 concentrations.