The green unicellular alga, Haematococcus pluvialis has two antioxidative mechanisms against environmental oxidative stress : antioxidative enzymes in vegetative cells and the antioxidative ketocarotenoid, astaxanthin, in cyst cells. We added a reagent that generates superoxide anion radicals (O-2(-)), methyl viologen, to mature and immature cysts of H. pluvialis. Tolerance to methyl viologen was higher in mature than in immature cysts. Mature (astaxanthin-rich) cysts showed high antioxidant activity against O-2(-) in permeabilized cells, but not in astaxanthin-free cell extracts, while immature (astaxanthin-poor) cysts had very low antioxidant activities against O-2(-) in both. The results suggested that astaxanthin accumulated in the cyst cells functions as an antioxidant against excessive oxidative stress. The same levels of antioxidant activities against O-2(-) in both permeabilized cells and cell extracts from vegetative cells suggested the presence of antioxidative enzymes (superoxide dismutase).