Aspergillus fumigatus produces substantial extracellular cellulases on several cellulosic substrates including simple sugars. Low glucose potentiates enzyme production, but most cellulose-induced cellulases are repressed by high glucose. As production of cellulase in a wide substrate range is unusual, the cellulolytic complex of this thermophilic fungus was investigated. A beta-glucosidase was separated by gel filtration and ion-exchange chromatography. It migrated in native polyacrylamide gel as a single protein (130 kDa), which split under denaturing conditions into two smaller proteins having molecular masses of 90 kDa and 45 kDa. However, only the 90-kDa protein was active. Conventional chromatographic procedures were unsuccessful for the separation of these two proteins. Therefore, the 130-kDa protein was studied for its kinetic properties. It hydrolyzed p-nitrophenyl-beta-D-glucopyranoside (p-NPG) and cellobiose, but not beta-glucans, laminarin, and p-nitrophenyl-beta-D-xilopyranoside. The optimal pH and temperature of p-NPG and cellobiose hydrolysis were 5.0 and 4.0, and 65 degrees C and 60 degrees C, respectively. The K-m values, determined for cellobiose and p-NPG of hydrolysis, were 0.075 mM and 1.36 mM, respectively. Glucose competitively inhibited the hydrolysis of p-NPG. The K-i was 3.5 mM.