A highly CO 2 -tolerant microalga, Chlorella sp., was isolated and its cultural characteristics were examined. Chlorella KR-1 showed excellent stabilities to high concentrations of CO 2 and high temperature. It maintained good growth in air containing up to 30% CO 2 and at temperatures up to 40 o 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-1 strain for mass cultivation using stack gases.