The dissimilatory sulphate-reducing bacterium Desulfovibrio gigas, frequently sub-cultured, often contained spherical granules which stained metachromatically with some basic dyes. The granules were examined in situ by transmission electron microscopy of whole organisms and thin sections. The granules were isolated from broken bacteria as a water-insoluble, non-crystalline, white material containing magnesium, phosphorus and organic carbon, but devoid of sulphur and nitrogen. The molar ratio of phosphorus to magnesium (1 to 17) was close to the proportions in magnesium tripolyphosphate. Infrared absorption spectra for the white material and magnesium tripolyphosphate were similar.