The mutant strain PN-120 of Cellulomonas flavigena produces a ss-glucosidase that is 10-fold more active than the corresponding enzyme isolated from the parental strain. These enzymes were partially purified through Q Sepharose and Bio-Gel filtration. A single protein band was detected on polyacrylamide-gel electrophoresis/zymogram using 4-methylumbelliferyl-beta-D-glucoside. On sodium dodecyl sulfate-PAGE, the enzyme displayed three protein bands, suggesting that in C. flavigena the enzyme is oligomeric with a molecular mass of 210 kDa. On purification, the specific activity of ss-glucosidase isolated from PN-120 was increased 16-fold and showed three times more affinity for cellobiose than the enzyme of the parental strain; nevertheless, the optimum pH and temperature were similar for both enzymes. The kinetic parameters suggested that the increase in the activity of the enzyme, from the mutant strain, was caused by a mutation that affects the catalytic site of the enzyme. The partial amino-acid sequence of the isolated enzyme confirmed that it is a beta-glucosidase because of its homology with other beta-glucosidases produced by cellulolytic bacteria and fungi.