The effects of solid substrates, initial moisture content, moistening medium, temperature and incubation time on xylanase production by Aspergillus niger DFR-5 was studied and the highest activity (2596 IU/g dry substrate (gds)) was achieved in medium that contained wheat bran (WB) and soybean cake (SBC) at a ratio of 70:30, was moistened to 70% with MSS-2 mineral salt solution, and incubated for 6 days at 40 degrees C. Water at 37 degrees C was suitable for efficient recovery of enzyme from moldy WB SBC medium. The extraction parameters for xylanase were optimized with respect to minimum volume of extractant using a central composite rotatable design (CCRD). The maximum recovery of xylanase (4465 +/- 52 IU/gds) with 92.5% desirability was obtained employing water (10 ml/gds) as extractant at 200 rpm for 60 min. The result shows that an overall 5.4-fold increase in xylanase production was obtained in concentrated form by optimizing medium components and extraction conditions. (C) 2010 Elsevier Ltd. All rights reserved.