Black meristematic fungi together with lichens and cyanobacteria dominate the micro-flora of rock surfaces in arid and semi-arid environments of hot and cold deserts. This study shows that rock inhabiting meristematic fungi are extremely tolerant against high temperatures, desiccation and osmotic stress. Their temperature tolerance increases with increasing dehydration of the fungal thallus. Air dried mycelia of black yeasts stand temperatures up to 120 °C for at least 0.5 hours. As response to high temperatures multilayered cell walls are developed and trehalose is accumulated whereas the intracellular glycerol regulates the osmotic potential under NaCl stress. Strains from rock in moderate climate (North Germany) show the same tolerance than those isolated from the Mediterranean area. Hortaea werneckii – hitherto only described as agent of human Tinea nigra – is shown to be the most tolerant rock inhabiting species tested. Meristematic fungi cannot be pre-adapted to higher growth temperatures by increased incubation temperatures. Considering the results of this study the justification of the term ’stress‘ is discussed with regard to rock inhabiting fungi and their natural environment. Consequences for conservation treatments of monuments decayed by meristematic fungi are discussed on the basis of the ecophysiological properties of the fungi.