The changes in trans-thylakoid membrane proton motive force caused by red light and caused by far-red light in the halotolerant green alga, Dunaliella salina are investigated. Irradiation with red light decreased the intensity of the fast phase of millisecond delayed light emission (ms-DLE) in D. salina, and far-red light led to the opposite effects. Under low temperature conditions (4 degrees C), red light still decreased ms-DLE fast phase intensity, however, far-red light did not enhance the ms-DLE fast phase intensity as it did at room temperature. In the presence of the uncoupler, nigericin, which eliminates the proton gradient across the thylakoid membrane, there was still a decrease in ms-DLE after red light irradiation, while far-red light had no stimulatory effects anymore. The far-red light-induced increase in ms-DLE fast phase is thus suggested to be due to the proton gradient formed by water oxidation in photosystem II. Previous studies with higher plants revealed that far red light increased ms-DLE fast phase intensity slightly, while red light caused a transient increase in ms-DLE fast phase intensity followed by a gradual decrease. Taken together, green algae differ from higher plants with respect to red light- and far red light-induced changes in ms-DLE. The possible reason is discussed.