A strain of Chlorella emersonii grown under high light intensity and low nitrogen degrades its chlorophyll and synthesizes canthaxanthin as the major carotenoid. Nitrogen starvation or high light alone does not induce canthaxanthin production. Norflurazon, a carotene inhibitor at the level of phytoene desaturase, inhibits production of canthaxanthin in C. emersonii with no accumulation of phytoene. Chlorella vulgaris and C. sp. 993 exposed to similar conditions do not respond in the same way as C. emersonii.