The synergistic actions of pairs of seven different exo- and endo-type cellulases produced by Irpex lacteus (Ex-1 and En-1), Trichoderma reesei (CBH I, CBH II, and Endo-2), and Aspergillus niger (Exo-A and EG-1) were investigated using pure cellulosic materials with different crystallinities as substrates. A marked synergism was observed with certain combinations of exo-type cellulases, namely, CBH II and the other exo-type cellulases (Ex-1, CBH I, or Exo-A), when saccharification activity against crystalline and amorphous celluloses was investigated. Differences between the modes of action of each exo-type cellulase alone and in combination are not clearly observed; the production of cellobiose was accelerated during the hydrolysis of crystalline and amorphous celluloses by the combinations of exo-type cellulases, and no effective synergistic actions of the combinations of exo-type cellulases were noted in the depolymerization of crystalline cellulose. However, the synergistic actions of exo-type cellulases with CBH II caused specific changes in the surface features of highly crystalline celluloses. Characteristic morphological changes in cotton fibers (erosion and cracking of fiber surfaces) were induced at increasing rates by the combination of CBH II and Ex-1 or Exe-A. In contrast, destruction and subdivision of fibrils, which resulted in the pronounced fibrillation of cotton fibers, were observed after the treatment of cotton fibers with CBH II and CBH I. It was suggested that CBH II may be one of the key enzymes involved in the synergistic actions during cellulose hydrolysis.