The change in the pigment composition of the unicellular alga Dunaliella parva 19/9 during exposure to high light (4000 μmol m(-2) s(-1)) has been investigated. During photobleaching the carotenoids were lost at a greater rate than the chlorophylls. In these photoinhibitory conditions, β-carotene and especially the minor carotenes, δ- and γ-carotene, were more susceptible to oxidative destriction than the xanthophylls. Lutein, the major carotenoid present, was the most stable of the carotenoids in these conditions. In addition to the direct photobleaching of carotenoids and chlorophylls, high light treatment induced the de-epoxidation of violaxanthin to antheraxantin and zeaxanthin. Small amounts of zeaxanthin were present in cells prior to illumination but the amount increased 2.4 fold following high light treatment. The effects of extremes of temperature during exposure to high light intensities were also investigated. The destruction of chlorophylls was found to be more temperature sensitive than that of the carotenoids. The general pattern of loss for the individual carotenoids was similar to that found at 25°C, i.e., the carotenes were more readily degraded than the xanthophylls.