A method for preparing a hyper-cellulolytic catabolite derepressed mutants of ascomycetes fungus, especially variants of Penicillium funiculosum. Selection media used to isolate such variants include amorphous cellulose and a high concentration of glucose. Cellulase activities of mutant ID-10, in particular such as FPase and β-glucosidase were 1.5 times higher than Penicillium funiculosum MRJ-16 (parent). Furthermore, fungal mutant morphology was changed and no pH adjustment was required throughout the enzyme production process.