Five different metal-substituted forms of Clostridium pasteurianum rubredoxin have been prepared and crystallized. The single Fe atom present in the Fe(S-Cys)(4) site of the native form of the protein was exchanged in turn for Co, Ni, Ga, Cd and Hg. All five forms of rubredoxin crystallized in space group R3 and were isomorphous with the native protein. The Co-, Ni- and Ga-substituted proteins exhibited metal sites with geometries similar to that of the Fe form (effective D(2d) local symmetry), as did the Cd and Hg proteins, but with a significant expansion of the metal-sulfur bond lengths. A knowledge of these structures contributes to a molecular understanding of the function of this simple iron-sulfur electron-transport protein.