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Composition of thermotolerant mycobiota in the soil of Israeli deserts and northern territories was examined in spatiotemporal dynamics. A total of 165 species from 82 genera were isolated at 37 degrees C using the soil dilution plate method. Aspergilli (Aspergillus fumigatus and A. niger) and teleomorphic ascomycetes (Canariomyces notabilis, Chaetomium nigricolor, and Ch. strumarium) comprised the basic part of the thermotolerant communities. The desert communities remarkably differed from the northern communities by a much higher abundance of A. fumigatus and teleomorphic species, as well as by a lower abundance of A. niger and Rhizopus arrhyzus. Seasonal dynamics revealed for the southern Negev was expressed mainly in the variations of species richness (substantially lower in the winter), and abundances of A. fumigatus (dominant in the summer) and A. niger (dominant in the winter). The composition of thermotolerant mycobiota was almost entirely different from the composition of mesophilic mycobiota at 25 degrees C. Melanin-containing fungi with many-celled conidia that dominated mesophilic communities in the deserts did not grow at 37 degrees C, while prevailing aspergilli accompanied by teleomorphic species with perithecial fruit bodies were apparently able not only to survive but also to germinate at this temperature and be active during a long hot period in the desert.