The cyst cells of Haematococcus pluvialis were separated into fractions of relatively uniform size by sucrose density gradient centrifugation. The fraction at the bottom of the centrifuge tube with the largest specific gravity from density gradients of mature cysts mainly consisted of enlarged, red cyst cells and had the highest astaxanthin content. To examine the relationship between cell size and astaxanthin content of cysts, formation of the fluorescent dichlorofluorescein (DCF) from 2',7'-dichlorohydrofluorescein diacetate of cyst cells in each fraction from density-gradient centrifugation under oxidative stress caused by methyl viologen (1.0 mM) was studied. The formation of DCF in cyst cells was decreased with larger cell diameter. This decrease was also correlated with increases in astaxanthin content. Therefore, both cell diameter and the fluorescent DCF content of cyst cells would be good parameter to select astaxanthin-hyperproducing strains from native populations of H. pluvialis.