The present-day energy crisis brings forth to attention biogas as an alternative energy source, i.e. economically feasible and unlimited in potential. Moreover, the economy of biogas plants can be improved by using high biogas potential substrates in combination with cattle dung. One such substrate was identified, having high nitrogen and phosphate content, to amend in cattle dung biogas plants. This amendment was mustard oil cake (MOC) (Brassica compestries) added at different ratios to cattle dung digesters. Results indicated a tremendous increase in volatile solids destruction, with a 12.2-13.08% increase in the case of 30% mustard cake addition with a corresponding gas production of 0.329 m(3) kg(-1) VSa with 4591 ml day(-1). This is almost a 63.44% increase in biogas production compared with only cattle dung, which resulted in 2809 ml day(-1). But based on the mustard cake availability, an optimum of 20% addition is also very suitable. In general, a biogas increase of 13.38%, 25.27%, 39.16%, 52.26% and 63.44% was observed at 10%, 15%, 20%, 25% and 30% mustard meal/cake addition. Capillary suction time (CST) remained in the range of 320-394s, indicating good waterability of the digested sludge. Manurial value of the digested sludge was very good. (c) 2007 Elsevier Ltd. All rights reserved.