Thermobifida fusca produces two exocellulases, Cel6B (non reducing end specific) and Cel48A (reducing end specific), which have been extensively characterized and a number of site directed mutations have been produced in Cel6B. These mutant enzymes have been characterized and some have unusual properties. Mutation of Asp226 to Ala decreased activity dramatically on amorphous and bacterial cellulose but not on CMC. Furthermore the products of CMC hydrolysis were dramatically changed by the mutation. Another unusual result is that several mutant enzymes had higher activity on crystalline cellulose but when they were tested in synergistic mixtures, they behaved like the wild type enzyme with no increase in their synergistic activity. A similar effect was found when Cel48A was compared to Clostridium thermocellum Cel48A. C. thermocellum Cel48A was four times more active on bacterial cellulose than T. fusca Cel48A but in synergistic mixtures T. fusca Cel48A gave slightly higher activity. Further work is needed to explain these surprising results. Azide rescue experiments carried out on potential catalytic base residue mutations in both T. fusca Cel6A and Cel6B did not find any evidence for a catalytic base suggesting that family 6 cellulases lack a catalytic base unlike other cellulase families.