The experiment was conducted to determine the effect on methane production of adding different proportions of fresh fruit waste to manure from pigs or buffaloes as substrate in plug-flow biodigesters. The treatments in a completely randomized 2*5 factorial design with with four replicates were ratios of fresh fruit waste to manure (pig or buffalo) of 0, 25, 50, 75 and 100 (% DM basis). Measurements were made of gas production, methane content of the gas and pH of the effluent over a period of 30 days from startup of the biodigesters. Gas production and the concentration of methane in the gas were increased by adding 25% of fruit waste to manure from buffaloes or pigs and then reduced dramatically when the fruit waste comprised 50% or more of the substrate entering the biodigester, the effect being much more dramatic when the manure component of the substrate was from buffaloes rather than pigs. The depressing effect on biodigester pH when fruit waste was the companion substrate was probably due to the high sugar content in the fruit waste.