An endo-beta-1,4-glucanase from a thermoacidophilic fungus, Aspergillus terreus M11, was purified 18-fold with 14% yield and a specific activity of 67 U mg(-1) protein. The optimal pH was 2 and the cellulase was stable from pH 2 to 5. The cellulase had a temperature optimum of 60 degrees C measured over 30 min and retained more than 60% of its activity after heating at 70 degrees C for 1 h. The molecular mass of the cellulase was about 25 kDa. Its activity was inhibited by 77% by Hg2+ (2 mM) and by 59% by Cu2+ (2 mM).