A gene encoding a thermo-stable endo-beta-1,4-glucanase was isolated from the thermophilic fungus, Thermoascus aurantiacus IFO9748, and designated as eg1. Induction of this gene expression at 50degreesC was stronger than at 30degreesC. The deduced amino acid sequence encoded by eg1 showed that it belongs to the glycoside hydrolase family 5. The cloned gene was expressed in Saccharomyces cerevisiae and the gene product was purified and characterized. No significant activity loss was detected over 2 h at 70degreesC and the product was stable from pH 3-10. The enzyme was optimally active at 70degreesC over 20 min and the optimal pH was 6.