A 56 kDa extracellular D-xylanase from mutant PN-120 of Cellulomonas flavigena was purified to homogeneity and characterized. The purified enzyme is an acidic protein with a pI of 6.14 and K-m, and V-max values for birchwood xylan of 0.43 mg/ml and 2500 IU/mg, respectively. The optimal temperature and pH for activity were 55 degreesC and 9, respectively. Homologous xylanase from wild strain exhibited identical characteristics in optimum temperature, pI and molecular mass however, optimum pH was 6.5 and K-m and V-max values were 1.27 mg/ml and 322 IU/mg, respectively. These results indicate that the mutational treatment apparently affected this xylanase on mutant PN-120.