In this study, two thermal-tolerant mutants of Chlorella sp. MT-7 and MT-15, were isolated. In indoor cultivation, specific growth rate (mu, d(-1)) of the mutants were 1.4 to 1.8-fold at 25 degrees C and 3.3 to 6.7-fold at 40 C higher than those of wild type. The carbon dioxide fixation rate of both microalgal mutants was also significantly higher than that of wild type. In outdoor closed cultivation, where the temperature of culture broth was 41 +/- 1 degrees C, the mu of mutant strain MT-15 was 0.238 d(-1) during an 8-day cultivation. Whereas, the growth of wild type was inhibited in the outdoor cultivation. Our results show that the isolated microalgal strains are adaptable to be applied in outdoor cultivation in subtropical zones. (C) 2009 Elsevier Ltd. All rights reserved.