Sequence comparisons between beta-glucosidases of the same family show that residues Glu183 and Glu397 are highly conserved. Both residues are positioned at the end of a pocket located at the C terminus of the barrel and have been assigned the respective roles of proton donor and nucleophile on the basis of inhibitor-binding and mutagenesis experiments. These roles are consistent with the environments of the two residues. The pocket itself is typical of a sugar-binding site as it contains a number of charged, aromatic and polar groups. In support of this role, we present crystallographic data on a possible product complex between CBG and glucose, resulting from co-crystallization of the native enzyme with its natural substrate, linamarin.