Methanosarcina are methanogens capable of growth and methanogenesis from H₂/CO₂, formate, methanol, methylamines, and acetate. Methanosarcina conserve energy by coupling electron transport and methyl transfer to the generation of ion gradients during acetoclastic growth. This work focuses on cytochrome b and heterodisulfide reductase, two proteins involved in energy conservation by electron transport. A procedure was developed for mass cultivation of Methanosarcina thermophila strain TM-1 in 12-liter fermentations which produced up to 10 grams wet weight/liter, in order to facilitate biochemical studies. Cytochromes occurring in Methanosarcina thermophila were characterized spectrophotometrically using chemical and physiological reactants. This analysis revealed two heme centers, one of which was only reduced by Na₂S₂O₄ or carbon monoxide. Partially purified cytochromes were found to be present in a complex and were characterized by electrophoretic and spectrophotometric analysis. The cytochrome-containing protein was found to contain two hemes and had an M_r of 28,000 Da. Heterodisulfide reductase was isolated from the soluble fraction by anion exchange chromatography and assayed using methyl viologen as an artificial electron donor. Electron transport from CO to the heterodisulfide of 2-mercaptoethanesulfonic acid (HS-CoM) and 7- mercaptoheptanoylthreonine phosphate (HS-HTP) was reconstituted using carbon monoxide dehydrogenase, ferredoxin, membranes, and heterodisulfide reductase. Both membranes and ferredoxin were required for reduction of the heterodisulfide.