The need for renewable energy resources has focused attention on the renewable polysaccharide cellulose, which can be enzymatically hydrolyzed to yield sugars. Trichoderma reesei QM9414, a cellulolytic microorganism, was subjected to mutagenesis using a combination of ultraviolet light and sodium nitrite (45 min, 0.5 mg ml(-1)). A cellulase hyperproducing mutant, whose enzyme is more resistant to catabolite repression by glucose and cellobiose, was isolated. Enhancement in cellulase yields (1.5- to 1.75-fold) and hydrolytic potential (1.2-fold) was achieved. The mild chemical mutagen used in this research has not been reported before.