Thermobifida fuscaexocellulase Cel6B acts by an inverting hydrolysis mech-anism; however, the catalytic acid and base residues for this enzyme have not been confirmed. Site-directed mutagenesis and kinetic studies were used to show that Asp274 is the catalytic acid, which is consistent with what is found for other members of family-6 glycoside hydrolases; however, a sin-gle catalytic base was not identified.