The gene 5 protein of filamentous bacteriophage fd is a single-stranded DNA-binding protein that binds non-specifically to all single-stranded nucleic acid sequences, but in addition is capable of specific binding to the sequence d(GT5G4CT4C) and the RNA equivalent r(GU5G4CU4C), the latter interaction being important for translational repression. We show that this sequence preference arises from the formation of a tetraplex structure held together by a central block of G-quartets, the structure of which persists in the complex with gene 5 protein. Binding of gene 5 protein to the tetraplex leads to formation of a ∼170 kDa nucleoprotein complex consisting of four oligonucleotide strands and eight gene 5 protein dimers, with a radius of gyration of 45 Å and an overall maximum dimension of 120–130 Å. A model of the complex is presented that is consistent with the data obtained. It is proposed that the G-quartet may act as a nucleation site for binding gene 5 protein to adjacent single-stranded regions, suggesting a novel mechanism for translational repression.