Cellobiohydrolase I (CBH I), the major component of Trichoderma reesei cellulolytic system, is comprised of a catalytic core domain joined to a cellulose binding-domain (CBD) by an extended O-glycosylated interdomain linker peptide. Two internal deletions were introduced to the linker in order to investigate its function particularly in the hydrolysis of crystalline cellulose. Deletion of the first one-third of the linker, including a putative hinge region, reduces the binding capacity of CBH I in high enzyme coverage but does not affect its enzymatic activity on crystalline cellulose. The longer deletion removing practically all of the linker dramatically reduces the rate of crystalline cellulose degradation even though the enzyme still binds to the substrate. We conclude that sufficient spatial separation of the two domains is required for efficient function of CBH I. It is evident that the presence of a functional CBD is increasingly important for CBH I toward higher enzyme to cellulose ratios. Our data suggest that the putative hinge removed by the first deletion facilitates CBD-driven binding and dense packing of the wild type enzyme on the cellulose surface.