A highly CO2-tolerant microalga, Chlorella sp., was isolated and its cultural characteristics were examined. Chlorella KR-I. showed excellent stabilities to high concentrations of CO2 and high temperature. It maintained good growth in air containing up to 30% CO2 and at temperatures up to 40 degrees C. The alga showed a good growth rate in a broad range of pH. Chlorella KR-1 was found to be suitable for high density culture. The results indicated the possibility of using the KR-I strain for mass cultivation using stack gases.