By light microscopy, Methanospirillum hungatei GP1 stains gram positive at the terminal ends of each multicellular filament and gram negative at all regions in between. This phenomenon was studied further by electron microscopy and energy-dispersive X-ray spectroscopy of Gram-stained cells, using a platinum compound to replace Gram's iodine (J. A. Davies, G. K. Anderson, T. J. Beveridge, and H. C. Clark, J. Bacteriol. 156:837-845, 1983). Crystal violet-platinum precipitates could be found only in the terminal cells of each filament, which suggested that the multilamellar plugs at the filament ends were involved with stain penetration. When sheaths were isolated by sodium dodecyl sulfate-dithiothreitol treatment, the end plugs could be ejected and their layers could be separated from one another by 0.1 M NaOH treatment. Each plug consisted of at least three individual layers; two were particulate and possessed 14.0-nm particles hexagonally arranged on their surfaces with a spacing of a = b = 18.0 nm, whereas the other was a netting of 12.5-nm holes with spacings and symmetry identical to those of the particulate layers. Optical diffraction and computer image reconstruction were used to clarify the structures of each layer in an intact plug and to provide a high-resolution image of their interdigitated structures. The holes through this composite were three to six times larger than those through the sheath. Accordingly, we propose that the terminal plugs of M. hungatei allow the access of larger solutes than does the sheath and that this is the reason why the end cells of each filament stain gram positive whereas more internal cells are gram negative. Intuitively, since the cell spacers which partition the cells from one another along the filament contain plugs identical in structure to terminal plugs, the diffusion of large solutes for these cells would be unidirectional along the filament-cell axis.