Trihaem cytochromec3 (also known as cytochrome and cytochromec7 ) is isolated fromthe periplasmic space of Desulfuromonas acetoxidans, a sulfur-reducing bacterium. Thermodynamic and kinetic data for the trihaem cyto-chromec3are presented and discussed in the context of the possible physiological implications of its functional proper-ties with respect to the natural habitat of D. acetoxidans, namelyas a symbiontwithgreensulfurbacteriaworkingas a mini-sulfuretum.