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Photosynthetic characteristics of Dunaliella salinawith high (red form) and low β-carotene (green form) concentrations were studied. D. salinagrowing in brine saltworks exhibited a high level of β-carotene (15 pg cell−1). The rate of oxygen evolution as a function of irradiance was higher in the red than in the green form (on chlorophyll basis). Photosynthetic inhibition of the green form was observed above 500 µmol m−2s−1. The red form appeared more resistant to high irradiance and no inhibition in O2evolution was observed up 2000 µmol m−2s−1. However, when these results are expressed on a cell number basis the rate of oxygen evolution was significantly higher in the green form.