The growth of Spirulina platensis was studied in a light-limited culture under various dissolved oxygen (DO) concentrations. At high DO concentration, e.g. at 1.25 mM DO, the growth rate was decreased up to 36 % compared with that of 0.063 mM DO. The retarded growth rate at high DO concentrations seemed to be coupled with the degeneration of photosynthetic activity in terms of O-2 evolution. Under higher DO concentrations, superoxide dismutase and ascorbate peroxidase activities tended to increase, Chile the contents of photosynthetic pigment, such phycocyanin, carotenoid and chlorophyll-a decreased distinctly,