The production of high titer xylanase without cellulase is required for prebleaching of pulps in pulp and paper industry. The mutant IITD3A of Melanocarpus albomyces developed from the spores of the wild type organism was used in this study. The statistical optimization of the process parameters by response surface methodology revealed that the production of xylanase was most affected by changes in the pH of the production medium which contained a soluble extract of wheat straw as the sole carbon source. When the pH of the production medium in a 14 L bioreactor was controlled on-line at 7.8, xylanase activity of 415 IU mL(-1) was obtained after 36 h fermentation. On cycling the pH between 7.8 and 8.2, the same activity could be attained in 24 h with an overall productivity of 16,670 IU L-1 h(-1). The production of xylanase was also influenced by the fungus morphology; the activity being maximum when it exhibited pellet form at an agitation speed of 600 rpm. On optimization of aeration rate to 0.25 vvm, the xylanase activity further increased to 550 IU mL(-1) with a very high overall volumetric productivity of 22,000 IU L-1 h(-1). Thus, a 5.2-fold enhancement in overall volumetric productivity of xylanase could be obtained by the mutant in comparison to that obtained on insoluble wheat straw. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.