The green unicellular alga,Haematococcus pluvialis has two antioxidative mechanisms against environmental oxidatative stress:antioxidative enzymes in vegevative cells and the antioxidative ketocarotenoid,astaxanthin,in cyst cells. We added a reagent that generates superoxide anion radicals（ O₂^-）methyl viologen, to mature and immature cysts of H .pluvials.Tolerance to methyl viologen was higher in mature than in immature cysts. Mature(astaxanthin-rich)cysts showed high antioxidant activity against  O₂^- in permeabilized cells, but not in astaxanthin-free cell extracts, while immature (astaxanthin-poor)cysts had very low antioxidant activities against O₂^- 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 o2 in both permeabilized cells and cell extracts from vegetative cells suggested the presence of antioxidative enzyment (superoxide dismutase).