Cellobiohydrolases (CBHs) hydrolyzing crystalline cellulose share a two-domain structure of catalytic domain (CD) and cellulose-binding domain (CBD). To focus on the binding characteristics of CBD, we analyzed the adsorption of fusion protein of fungal family 1 CBD from Trichoderma reesei CBH I and red-fluorescent protein on crystalline and amorphous celluloses. Binding data were better fitted by Hill's model with negative cooperativity than by other adsorption models, suggesting the occurrence of a steric exclusion effect among the fusion molecules on the cellulose surfaces. The degree of negative cooperativity depended on the nature of the cellulose. The significance of this phenomenon for catalysis by intact CBHI is discussed.