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1. In previous studies it was shown that, in contrast to green algal lichens and aerophilic green algae, dry cyanobacterial lichens and some free-living cyanobacteria which were also investigated were not able to reactivate photosynthetic metabolism solely by hydration in equilibrium with high air humidity. They needed liquid water to attain positive net photosynthesis. The same proved to be true for the cyanobacterium Microcoleus sociatus in a soil crust of the Negev Desert when it was studied in its natural summer state of development. 2. In contrast, desiccated populations of the same organism in laboratory cultures attained turgor and were able to photosynthesize in equilibrium with a relative air humidity as low as 96% (15 degrees C). They reached about 30% of the maximum net photosynthesis without liquid water in almost water-saturated air. Rates of photosynthesis were similar and no hysteresis effects occurred, when equilibrium in water potential with high air humidity was approached from the dry or from the wetted state. 3. This study shows that water vapour activation of net photosynthesis can not be attributed to green algae and to green algal lichens alone. This capacity might be also important for productivity of desert soil crusts which are formed by cyanobacteria.