The precursor polypeptides of periplasmic proteins binding seven types of redox cofactor have unusually long signal sequences bearing a consensus (S/T)‐R‐R‐x‐F‐L‐K motif immediately before the hydrophobic region. Such ‘double‐arginine’ signal sequences are not, in general, found on the precursors of other periplasmic proteins. It is suggested that precursor proteins with double‐arginine signal sequences share a common specialization in their export pathway. The nature of this specialization, the structure of the double‐arginine signal sequences, and the possible relationship with the double‐arginine signal peptide‐dependent thylakoid import pathway are discussed.