In contrast to green algal lichens, cyanobacterial species of different families, growth forms and habitats proved to be unable to attain positive net CO2 assimilation when the dry thalli were treated with air of high relative humidity; they needed liquid water for the reactivation of their photosynthetic apparatus. Identical behaviour is shown by all of the 47 lichen species with cyanobacterial photobionts, from six different genera, studied so far. This suggests a widely distributed, if not general, characteristic of cyanobacterial lichens. The difference in performance between both groups of photobionts was maintained when the lichen thallus was macerated. Furthermore, cultures of Chroococcidiopsis were unable to make use of water vapour hydration for positive net photosynthesis, and were similar in this respect to some free-living aerophilic cyanobacteria tested earlier. Possible physiological implications as well as ecological consequences for water-relation-dependent habitat selection of green-algal and cyanobacterial lichens are discussed.