The integral membrane protein fumarate reductase catalyzes the final step of anaerobic respiration when fumarate is the terminal electron acceptor. The homologous enzyme succinate dehydrogenase also plays a prominent role in cellular energetics as a member of the Krebs cycle and as complex II of the aerobic respiratory chain. Fumarate reductase consists of four subunits that contain a covalently linked flavin adenine dinucleotide, three different iron-sulfur clusters, and at least two quinones. The crystal structure of intact fumarate reductase has been solved at 3.3 angstrom resolution and demonstrates that the cofactors are arranged in a nearly linear manner from the membrane-bound quinone to the active site flavin. Although fumarate reductase is not associated with any proton-pumping function, the two quinones are positioned on opposite sides of the membrane in an arrangement similar to that of the Q-cycle organization observed for cytochrome bc1.