Free trans-astaxanthin accumulated in the alga Chlorococcum sp. was markedly enhanced from 3.664 mg g(-1) cell dry weight to 5.724 mg g (1) cell dry weight when the culture was supplemented with hydrogen peroxide (0.1 mM) under mixotrophic conditions of growth. After saponification, a total of 7.086 mg astaxanthin per g cell dry weight was achieved. Similarly, in heterotrophic cultures, the total astaxanthin content was increased from 1.034 mg g (1) cell dry weight without H2O2 to 1.782 mg g(-1) cell dry weight with 0.1 mM H2O2. Results indicate that hydrogen peroxide effectively induces the formation of free trans-astaxanthin in Chlorococcum sp.