The use of congress grass (Parthenium sp.) and water hyacinth (Eichhornia crassipes) as low cost raw materials for xylanase production from mutant Penicillium oxalicum SAU(E)-3.510 in submerged fermentation was investigated. For development of mutant from wild type P. oxalicum SA-8ITCC 6024, a strategy of mixed mutagenesis was followed using UV-irradiation and ethidium bromide, which had resulted into 1.87 fold increases in the activity of the enzyme. For enzyme production, the fungus was cultivated in mineral medium containing congress grass as carbon source. Considerably higher levels of production (475.2 +/-6.0 IU ml(-1)) were achieved in media containing congress grass, although it was slightly less than that was obtained (488.5 +/- 6.5 IU ml(-1)) in presence of commercial oat spelt xylan. This fact confirms the feasibility of using this low cost non-food resource as an alternative carbon source to save costs of the enzyme production process. Maximum xylanase activity was reported at 55 degrees C with its stability at 80 degrees C for 2 h. The highest activity of xylanase at pH 9.0 and its stability at similar pH for 24 h denote the alkalitolerant nature of enzyme. (C) 2008 Elsevier Ltd. All rights reserved.