The halotolerant green alga Dunaliella bardawil is known to accumulate β-carotene in response to stress factors such as high light intensity, high salt concentrations and nutrient limitation. In this report, the accumulation of βcarotene was studied in cells from nitrate-limited chemostat cultures, in comparison with those of D. salina, a strain that does not accumulate β-carotene under stress conditions. D. bardawil responded to growth arrest by accumulating β-carotene and, to a lesser degree, lutein and zeaxanthin. A substantial fraction of β-carotene and all the lutein and zeaxanthin was associated with the thylakoid fraction. The accumulation of carotenoids in D. bardawil occurred only in the light, but the light intensities were far below those where the photosynthetic rate is maximal. After growth arrest, the amount of chlorophyll (Chi) decreased in both strains. However, in D. bardawil Chi a decreased to a lesser extent in comparison with Chi b, which resulted in an increased Chi alb ratio. The maximum photosynthetic capacity declined rapidly in both strains after growth arrest. In contrast, the photosynthetic efficiency showed a temporary increase in D. bardawil and a decrease in D. salina. This increase did not occur when carotenogenesis was inhibited by diphenylamine, implying a causal relationship between enhanced carotenogenesis and increase of photosynthetic efficiency. The possible involvement of stress-accumulated carotenoids in photosynthetic activity is discussed.