A novel mesophilic hydrogen-producing bacterium was isolated from cow dung compost and designated as Clostridium sp. HR-1 by 16S rRNA gene sequence. The optimum condition for hydrogen production by strain HR-1 was pH of 6.5, temperature of 37 degrees C and yeast extract as nitrogen sources. The strain HR-1 has the ability to utilize kinds of hexose and pentose as carbon sources for growth and H(2) production. Cell growth and hydrogen productivity were investigated for batch fermentation on media containing different ratios of xylose and glucose. Glucose was the preferred substrate in the glucose and xylose mixtures. The high glucose fraction had higher cell biomass production rate. The rate of glucose consumption was higher than xylose consumption, and remained essentially constant independent of xylose content of the mixture. The rate of xylose utilization was decreased with increasing of the glucose fraction. The average H(2) yield and specific H(2) production rates with xylose and glucose are 1.63 mol-H(2)/mol xylose and 11.14-H(2) mmol/h g-cdw, and 2.02 mol-H(2)/mol-glucose and 9.37 mmol-H(2)/h g-cdw, respectively. Using the same initial substrate concentration, the maximum average H(2) yield and specific H(2) production rates with the mixtures of 9 g/l xylose and 3 g/l glucose was 2.01 mol-H(2)/mol-mixed sugar and 12.56 mmol-H(2)/h g-cdw, respectively. During the fermentation, the main soluble microbial products were ethanol and acetate which showed trends with the different ratios of xylose and glucose. (C) 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.