Protoplasts of the monokaryotic strain from hyperligninolytic fungus IZU-154 were treated with UV irradiation, and the regenerants were screened for their inability to degrade Walseth cellulose (WC) and carboxymetylcellulose (CMC) on an a,oar plate. From 2700 regenerants, cellulase-deficient and noncellulase mutants were isolated and designated Cel-139 and Cel-145, respectively. Cel-139 showed 15%, Avicelase, 60% CMCase, and 10% beta-glucosidase activities; however, it retained the same level of lignin degrading capability and manganese :peroxidase (MnP) production when compared with wild-type IZU-154. On the other hand, Cel-145 showed deterioration in lignin-degrading capability, despite a substantial level of MnP production. Furthermore, biobleaching of hardwood kraft pulp (HWKP) by wild-type IZU-154 and Cel-139 in the solid-state fermentation system, without supplemental nutrients, were investigated. After 5 d of treatments with both fungi, pulp brightness increased from 33 to 65% ISO brightness, and Kappa number decreased from 13.9 to 6.0. However, yield loss of biobleached pulp was 20% lower with Cel-139 than with wild-type IZU-154.