A thermophilic cyanobacterium, Chroococcidiopsis sp. strain TS-821 was cultivated under various CO2 concentrations using 1-l photobioreactor. The strain can grow well under aeration without addition of any extra CO2. Addition of 5 or 10% CO2 in the flow gas did not show significant effects on the growth of the strain, but aeration with a gas mixture containing 15 or 20% CO2 inhibited the growth. RubisCO activity in the cells showed higher values when cultivated under 5 or 10% CO2 than when cultivated under air, but those values under 15 or 20% CO2 were very low. Little difference was observed in the final amount of RubisCO protein in the cells cultivated under different CO2 concentration conditions. This indicated that RubisCO activity was controlled not transcriptionally but post-translationally by change in CO2 concentration during cultivation.