Electron microscopy has been used to study the morphology of Methanosarcina mazei and Methanobacterium thermoautotrophicum. Internal membranes were shown to be fixation artifacts. Immunogold techniques were used to identify bacterial species in anaerobic biomass digesters and to determine the cellular locations of enzymes important in methanogenesis. Invasion of and destruction of the cell wall of plant tissue in a digester by cellulolytic bacteria are also illustrated.