A gene encoding an endo-beta-1,4-glucanase, which is highly resistant to high temperature, protease and surfactant treatment, was isolated from Aspergillus niger IFO31125 and designated as eng1. The deduced amino acid sequence encoded by eng1 showed high homology with the sequence of a not-well-characterized cellulase encoded by eglB which has not yet been shown to be a stable enzyme. To confirm the sequence of the gene encoding the highly stable endo-beta-1,4-glucanase, the cloned gene was expressed in the yeast Saccharomyces cerevisiae, in which no cellulase activity was found, and the gene product was purified and subjected to enzymatic characterization. The enzyme retained 56% of the initial activity after 1 h of incubation at 80degreesC and was stable in the range of pH 3.0-10.0. The optimal temperature for enzyme activity was 70degreesC and the optimal pH was 6.0. The enzyme was highly protease-resistant and retained more than 80% of the initial activity after protease treatment for 3 d at 40degreesC. The enzyme was also resistant to various surfactants. From these results, eng1 was confirmed to encode a very stable endo-beta-1,4-glucanase.