Aim: Bioaugumentation of low temperature biogas production was attempted by addition of cold-adapted Clostridium and a methanogen. Methods and Results: A psychrotrophic xylanolytic acetogenic strain Clostridium sp. PXYL1 growing optimally at 20 degrees C and pH 5 center dot 3 and a Methanosarcina strain, PMET1, growing optimally on acetate and producing methane at 15 degrees C were isolated from a cattle manure digester. Anaerobic conversion of xylose at 15 degrees C with the coculture of the two strains was performed, and batch culture methane production characteristics indicated that methanogenesis occurred via acetate through 'acetoclastic' pathway. Stimulation studies were also undertaken to evaluate the effect of exogenous addition of the coculture on biogas yields at 15 degrees C. Addition of 3 ml of PXYL1 at the rate of 12 x 102 CFU ml-1 increased the biogas 1 center dot 7-fold (33 l per kg cowdung) when compared to control (19 center dot 3 l per kg cowdung) as well as increased the volatile fatty acid (VFA) levels to 3210 mg l-1 when compared to 1140 mg l-1 in controls. Exogenous of addition of 10 ml PMET1 inoculum at the rate of 6 center dot 8 +/- 102 CFU ml-1 in addition to PXYL1 served to further improve the biogas yields to 46 l kg-1 as well as significantly brought down the VFA levels to 1350 mg l-1. Conclusions: Our results suggest that the rate-limiting methanogenic step at low temperatures could be overcome and that biogas yields improved by manipulating the population of the acetoclastic methanogens. Significance and Impact of the Study: Stimulation of biomethanation at low temperature by coculture.