The Central Negev Desert and the Arava Valley, two closely situated geographical regions in the southern part of Israel, are areas of extreme desert characteristics. Both are exposed to sporadic, as well as regularly fluctuating environmental condi tions. They differ, however, with respect to the amount of precipitation, average temperature, average relative humidity, number of dew nights and light intensity (Table 1). From Table 1 it is obvious that, although both regions are inhospitable habitats, any organism living in the Arava Valley must tolerate more formidable conditions than those prevailing in the Central Negev. A comparison between the two floras shows that the two regions are inhabited by completely different lichen populations. In the Arava Valley, the flora consists almost entirely of cyanolichens belonging to the Lichinaceae, Heppiaceae and Col- lemataceae (Marton, 1977), whereas in the Central Negev, lichens with eukaryotic phycobionts predominate (Galun, 1970). The ability to withstand the more extreme conditions previlaing in the Arava Valley evidently depends on some basic properties of the cyanophilous lichens. We shall therefore compare between some basic metabolic processes and structural fea tures of these two lichen populations and their phycobionts. The existence, and survival, of free-living cyanobacteria in extreme environments has been amply documented (Shilo, 1979; Friedmann & Galun, 1974), The remark able ability of cyanolichens to survive and grow in the extreme conditions of the Arava Valley may stem not only from inherent characteristics which allow for cyanobacterial survival, but also from characteristics expressed as a result of lichenization.