A total of 500 strains of the thermophilic fungus Thermomyces lanuginosus were screened for their ability to produce proteases. A selected strain, T. lanuginosus P134, showed high enzymatic activity for proteases. The maximal yield of proteases was obtained with 4% casein, 4% glucose and 4% yeast extract in submerged culture. The protease system was shown to contain at least two protease isoenzymes by SDS–PAGE with gels containing gelatin as protease substrate. The effects of inhibitors imply that the two enzymes appear to be serine proteases. The crude proteases of T. lanuginosus P134 displayed unusual properties. The proteases were shown to be one of the most thermostable proteases so far isolated in fungi. The proteases were 100% stable at 50 °C. The half-life at 60 and 70° was approximately 160 min and 60 min, respectively. Temperature optimum of activity was 70°. The crude proteases were stable over a broad pH range (4–11). Optimum pH of activity was at 5·0 and 9·0.