When cells of the halotolerant cyanobacterium, Aphanothece halophytica were transferred from control culture medium that contained 0.5 M NaCl to a hypersaline medium with either 1.5 M or 2.0 M NaCl, the rate of photosynthetic CO2 fixation fell instantaneously. Subsequently, the photosynthetic activity returned to almost the original level within 1 day. Under salt stress, ultrastructural changes in the cells were observed by electron microscopy, these cells appeared to recover in parallel with the recovery of their photosynthetic activity. However, the intracellular level of glycinebetaine increased more slowly than the recovery of the photosynthetic CO2 fixation. The maximum rate of accumulation of betaine was estimated to be approximately 60 nmol (mg protein)-1 h-1. This rate is at least one order of magnitude greater than rates reported previously in leaves of salt-stressed higher plants. The accumulation of betaine did not take place in the dark. The intracellular level of betaine decreased as a result of the transfer of the cells to a hypo-osmotic NaCl-containing medium. The accumulation of betaine was also induced by an organic osmoticum, sorbitol. Nitrate enhances the accumulation of betaine under salt stress.