SUMMARYThermophilic and thermotolerant fungi were isolated from samples of domestic and municipal compost piles, self-heated industrial wood chip piles, alligator nests, sun-heated plant debris and soil, tree bark piles, sun-heated herbivore dung, steam line discharge sites, self-heated coal waste piles, an incubator bird nest, geothermal soils, and hot springs using lignin-containing enrichment media. The lignin added was Indulin AT (Westvaco), which is a Kraft lignin. The lignin-degrading ability of these isolates and of other thermophilic and thermotolerant fungi was assayed using clearing of lignin in agar, change in dry weight of liquid cultures containing lignin, and absorbance of culture filtrates at 280 and 460 nm in 1 N NaOH. None of the fungi tested cleared lignin. None of the isolates caused more than 9% loss of dry weight when grown on lignin in liquid culture. Decrease in absorption of culture filtrates at 280 and 460 nm accompanied increase in dry weight in a lignin-containing medium. Major changes in absorbance were observed in a different lignin-containing medium, but the changes were not consistent with changes in dry weight. Data are presented for growth at 37 C and SO C and for 14 da and 60 da.