The structure of Wolinella succinogenes quinol:fumarate reductase by X‐ray crystallography has been determined at 2.2‐Å resolution [Lancaster et al. (1999), Nature 402, 377–385]. Based on the structure of the three protein subunits A, B, and C and the arrangement of the six prosthetic groups (a covalently bound FAD, three iron–sulphur clusters, and two haem b groups) a pathway of electron transfer from the quinol‐oxidising dihaem cytochrome b in the membrane to the site of fumarate reduction in the hydrophilic subunit A has been proposed. By combining the results from site‐directed mutagenesis, functional and electrochemical characterisation, and X‐ray crystallography, a residue was identified which is essential for menaquinol oxidation. [Lancaster et al. (2000), Proc. Natl. Acad. Sci. USA 97, 13051–13056]. The location of this residue in the structure suggests that the coupling of the oxidation of menaquinol to the reduction of fumarate in dihaem‐containing succinate:quinone oxidoreductases could be associated with the generation of a transmembrane electrochemical potential. Based on crystallographic analysis of three different crystal forms of the enzyme and the results from site‐directed mutagenesis, we have derived a mechanism of fumarate reduction and succinate oxidation [Lancaster et al. (2001) Eur. J. Biochem. 268, 1820–1827], which should be generally relevant throughout the superfamily of succinate:quinone oxidoreductases.