Prokaryotic viruses have evolved various mechanisms to transport their genomes across bacterial cell walls(1). Many bacteriophages use a tail to perform this function, whereas tail-less phages rely on host organelles(2-4). However, the tail-less, icosahedral, single-stranded DNA Phi X174-like coliphages do not fall into these well-defined infection processes. For these phages, DNA delivery requires a DNA pilot protein(5). Here we show that the Phi X174 pilot protein H oligomerizes to forma tube whose function is most probably to deliver the DNA genome across the host's periplasmic space to the cytoplasm. The 2.4 angstrom resolution crystal structure of the in vitro assembled H protein's central domain consists of a 170 angstrom-long alpha-helical barrel. The tube is constructed of ten alpha-helices with their amino termini arrayed in a right-handed super-helical coiled-coil and their carboxy termini arrayed in a left-handed super-helical coiled-coil. Genetic and biochemical studies demonstrate that the tube is essential for infectivity but does not affect in vivo virus assembly. Cryo-electron tomograms show that tubes span the periplasmic space and are present while the genome is being delivered into the host cell's cytoplasm. Both ends of the H protein contain transmembrane domains, which anchor the assembled tubes into the inner and outer cell membranes. The central channel of the H-protein tube is lined with amide and guanidinium side chains. This may be a general property of viral DNA conduits and is likely to be critical for efficient genome translocation into the host.