When a green alga Haematococcus pluvialis was cultivated at 30 degrees C, astaxanthin production was 3-fold more increased than at 20 degrees C. With acetate supplementation to 30 degrees C culture, the alga synthesized over 2-fold more carotenoid than without addition. Tiron, a radical scavenger, however, severely blocked the stimulated carotenogenesis, suggesting that endogenously generated active oxygen was responsible for the highly stimulated carotenogenesis. From these results, possible roles of the elevated cultivation temperatures for hyperaccumulation of astaxanthin were discussed.