The fine structure of the photosynthetic bacterium Rhodomicrobium vannielii was studied by the ultra thin sectioning technique. Cells were fixed in buffered osmium tetroxide and embedded in Epoxy resin. The feature most common to nearly all cells was an array of intracellular membranes situated in a concentric manner at the periphery of the cell. The membranes were mostly paired and quite often five pairs were seen aligned together. Calculations from densitometric tracings showed the average width of a "unit" membrane to be 65 A. Sections of material from disrupted cells after passage through a sucrose gradient revealed vesicular forms composed of membranes similar in width to those in the intact cell. Absorption spectra of both intact cells and isolated membranes were very similar in the bacteriochlorophyll regions. Septa and membranes were demonstrated in the filaments that join mature cells. No evidence for chromatophores was obtained although the methods used were adequate for their demonstration in Rhodospirillum rubrum.

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