Reduction of non-native protein disulphides in the periplasm of Escherichia coli is catalysed by three enzymes, DsbC, DsbG and DsbE, each of which harbours a catalytic Cys-X-X-Cys dithiol motif. This dithiol motif requires continuous reduction for activity. Genetic evidence suggests that the source of periplasmic reducing power resides within the cytoplasm, provided by thioredoxin (trxA) and thioredoxin reductase (trxB). Cytoplasmic electrons donated by thioredoxin are thought to be transferred into the periplasm via the DsbD membrane protein. To understand the molecular nature of electron transfer, we have analysed the membrane topology of DsbD. DsbD is exported by an N-terminal signal peptide. The N- and C-terminal domains are positioned in the periplasmic space, connected by eight transmembrane segments. Electron transfer was shown to require five cysteine sulphydryl of DsbD. Trans complementation of mutant DsbD molecules revealed intermolecular electron transfer. We discuss a model whereby the membrane-embedded disulphides of DsbD accept electrons from cytoplasmic thioredoxin and transfer them to the C-terminal periplasmic dithiol motif of DsbD.