Fumarate reductase couples the reduction of fumarate to succinate to the oxidation of quinol to quinone, in a reaction opposite to that catalysed by the related complex II of the respiratory chain (succinate dehydrogenase). Here we describe the crystal structure at 2.2 A resolution of the three protein subunits containing fumarate reductase from the anaerobic bacterium Wolinella succinogenes. Subunit A contains the site of fumarate reduction and a covalently bound flavin adenine dinucleotide prosthetic group. Subunit B contains three iron-sulphur centres. The menaquinol-oxidizing subunit C consists of five membrane-spanning, primarily helical segments and binds two haem b molecules. On the basis of the structure, we propose a pathway of electron transfer from the dihaem cytochrome b to the site of fumarate reduction and a mechanism of fumarate reduction. The relative orientations of the soluble and membrane-embedded subunits of succinate:quinone oxidoreductases appear to be unique.