The synergism between the endo/exocellulase, Cel9A, and beta-glucosidase (beta gl) of Thermobifida fusca was investigated. Wild type beta gl or S319C, a beta gl mutant with significantly improved cellobiase activity, were added to Cel9A. Both wild type and mutant beta gl enhanced the Cel9A hydrolysis of carboxymethyl cellulose (CMC) and filter paper by 50-100% compared to Cel9A alone. No enhancement occurred with addition of E388A, an inactive form of beta gl. HPLC analysis showed that, with Cel9A alone, the resulting hydrolysate of glucose and cellobiose contained about half glucose; after addition of equimolar amounts of either wild type beta gl or mutant S319C to Cel9A, the hydrolysate contained more than 85% glucose. beta gl thus acted synergistically with Cel9A by converting cello-oligomers to glucose; this reduced the soluble sugar accumulation during hydrolysis of cellulose.