Digestion of Jatropha curcas seed cake was investigated in two-stage (acid ogenic and methanogenic) anaerobic bioreactors without pH adjustment. Acidogenic reactors were fed once daily with a slurry of 1:10 Jatropha curcas seed cake:water containing approximately 100 g of chemical oxygen demand/l. Organic loading rates were 2.5, 3.3, 5, 10, and 20 kg chemical oxygen demand/m(3).day, which corresponded to hydraulic retention times of 40, 30, 20, 10, and 5 days, respectively, for each reactor stage. The maximum methane yield (340 l at STP/kg of chemical oxygen demand degraded) was observed at an organic loading rate of 3.3 kg chemical oxygen demand/m(3).day (hydraulic retention times = 30 days for each stage). At this organic loading rate and hydraulic retention time, the chemical oxygen demand degradation efficiency was 65%. The average pH in the acidogenic and methanogenic reactors was 4.9 and 7.4, respectively. This study demonstrates high methane yield and degradation extent of Jatropha curcas seed cake in a two-stage anaerobic process without chemical addition for pH adjustment.