Cellulolytic enzymes consist of distinct catalytic and cellulose-binding domains (CBDs). The presence of a CBD improves the binding and activity of cellulases on insoluble substrates but has no influence on their activities on soluble substrates. Structural and biochemical studies of a fungal CBD from Trichoderma reesei cellobiohydrolase I have revealed a wedge shaped structure with a flat cellulose binding surface containing three essential tyrosine residues. The face of the wedge is strictly conserved in all fungal CBDs while many differences occur on the other face of the wedge. Here we have studied the importance of these differences on the function of the T. reesei CBHI by replacing its CBD by a homologous CBD from the endoglucanase, EGI. Our data shows that, apart from slightly improved affinity of the hybrid enzyme, the domain exchange does not significantly influence the function of CBHI.