A series of experiments was conducted to determine the performance characteristics of the methane fermentation process using beef feed lot manure as a substrate. Manure was obtained from the University of Illinois beef farm. This manure was processed through four parallel fermentors each having a capacity of 775 liters. A continuous feed system was employed to determine the conversion efficiency. The effluent from the fermentation units was evaluated to determine its dewatering characteristics and the quality of the liquid and solid residues. A simple simulation model was developed to evaluate the effect of various operating conditions on processing costs and the net income. These studies clearly show that thermophilic fermentation (58 to 60/sup 0/C) substantially increase the gas yield and the rate of gas production over that obtained at the mesophilic fermentation temperature. System stability is very good. Substantial decreases in temperature or significant increases in loadings did not disrupt the process. Solids recovery from the fermented slurry was accomplished with screens, vacuum drum filters and centrifuge. Solids capture was poor unless massive dosags of conditioning chemicals were added. In terms of investment and operating costs, simple screens (20 mesh size) would capture 75 to 80 percent of the recoverable suspended solids. Manure that is obtained from open lots, especially when it has been exposed to the environment for extended periods, offers little potential for methane production. The biodegradability of this material is so low that the cost of producing the gas far exceeds its value. Fresh manure such as that obtained from environmental lots produced significant quantities of gas. It is probable that an economic system can be developed using this material as a substrate.