The anaerobic digestion of domestic solid waste offers an alternative source for supplemental amounts of fuel gas. Using shredded newsprint and primary sewage sludge, we have examined the use of anaerobic digestion at a thermophilic temperature (55°C) for methane production. The maximum productivity occurred at a nominal retention time of 5 days using a total solids feed of 50g/l. The rate of gas production also correlates with the ratio of propionic plus butyric acids to acetic acid in the cell-free broth; this correlation is hypothesized to be the result of higher yields of methane from anaerobic utilization of the longer-chain fatty acids. The methane content of the gas depends on the liquid flow rate; carbon dioxide is much more soluble in water than methane and, as the liquid flow rate is increased, more CO2 is removed in the liquid phase, thus enriching the methane in the gas phase.