The growth rate of a highly CO2-tolerant green alga, Chlorococcum littorale, was investigated in semi-batch cultures at a temperature of 22 degrees C, a light intensity of 170 mu mol-photon m(-2) s(-1) and CO2 concentrations ranging from 1 to 50% (v/v) at atmospheric pressure. In the experiments, solutions were bubbled with CO2 and N-2 gas mixtures to adjust CO2 concentrations to minimize the influence Of O-2. Growth rate, which was defined in terms of a specific growth rate p, decreased with increasing CO, concentration at the conditions studied. The inhibition of growth by CO2 gas could be attributed to the concentration of inorganic carbon in the culture medium. A growth model is proposed where key assumptions are the formation of bicarbonate ion HCO3- as substrate for algal growth and equilibrium between CO2 inhibitor. The proposed growth model based on the Monod equation agreed with the experimental data to within 5% and provides better correlation than the conventional inhibition model, especially in the high CO2 concentration region. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 25: 492-498, 2009