Acetabularia mediterranea algae, grown in three different light-dark regimes, were frozen in liquid nitrogen at c.t.(1) 0 and c.t. 6 and a record made of 77 degrees K fluorescence emission spectra of their chloroplasts. Algae grown under LD cycles exhibited a clear circadian rhythm of oxygen production. The low temperature fluorescence emission spectrum at c.t.0 was different from that at c.t.6 and this difference was increased by submitting the algae to successive "freeze-thaw" treatment. Similar results were obtained in DD, and the photosynthesis rhythm remained fully expressed. Algae grown in LL, where no rhythm of photosynthesis could be detected in the samples because there is a great individual variability in period lenght under these conditions, exhibited a similar difference in their low temperature flourescence emission spectra between c.t.0 and c.t.6. We conclude that the circadian rhythm in low-temperature fluorescence emission of the chloroplasts in Acetabularia is related to the circadian rhythm in photosynthesis.