Sulfur particles extracted from Chromatium vinosum strain D were found to be bounded by a unique proteinaceous membrane. Ultrastructural examination of the membrane in Epon sections and bovine serum albumin sections and examination of negatively stained, sulfur-free membrane ghosts revealed a monomolecular sheet composed of 2.5-nm globular components. The internal sulfur was found to bind large amounts of a variety of negative stains and to form myelin-like structures upon rupture of the surrounding membrane.