Long-term acclimation to irradiance stress (HL) of the green alga Dunaliella salina Teod. (UTEX 1644) entails substantial accumulation of zeaxanthin along with a lowering in the relative amount of other pigments, including chlorophylls and several carotenoids. This phenomenon was investigated with wild type and the zea1 mutant of D. salina, grown under conditions of low irradiance (LL), or upon acclimation to irradiance stress (HL). In the wild type, the zeaxanthin to chlorophyll (Zea/Chl)(mol : mol) ratio was as low as 0.009 : 1 under LL and as high as 0.8 : 1 under HL conditions. In the zea1 mutant, which constitutively accumulates zeaxanthin and lacks antheraxanthin, violaxanthin and neoxanthin, the Zea/Chl ratio was 0.15 : 1 in LL and 0.57 : 1 in HL. The divergent Zea/Chl ratios were reflected in the coloration of the cells, which were green under LL and yellow under HL. In LL-grown cells, all carotenoids occurred in structural association with the Chl-protein complexes. This was clearly not the case in the HL-acclimated cells. A beta-carotene-rich fraction occurred as loosely bound to the thylakoid membrane and was readily isolated by flotation following mechanical disruption of D. salina. A zeaxanthin-rich fraction was specifically isolated, upon mild surfactant treatment and differential centrifugation, from the thylakoid membrane of either HL wild type or HL-zea1 mutant. Such differential extraction of beta-carotene and Zea, and their separation from the Chl-proteins, could not be obtained from the LL-grown wild type, although small amounts of Zea could still be differentially extracted from the LL-grown zea1 strain. It is concluded that, in LL-grown D. salina, xanthophylls (including most of Zea in the zea1 strain) are structurally associated with and stabilized by the Chl-proteins in the thylakoid membrane. Under HL-growth conditions, however, zeaxanthin appears to be embedded in the lipid bilayer, or in a domain of the chloroplast thylakoids that can easily be separated from the Chl-proteins upon mild surfactant treatment. In conclusion, this work provides biochemical evidence for the domain localization of accumulated zeaxanthin under irradiance-stress conditions in green algae, and establishes protocols for the differential extraction of this high-value pigment from the green alga D. salina.