The helical path of the DNA in filamentous bacteriophage Pf1 was deduced from different kinds of existing structural information, including results from x-ray fiber diffraction. The DNA has the same pitch, 16 angstroms, as the surrounding helix of protein subunits; the rise and rotation per nucleotide are 6.1 angstroms and 132 degrees, respectively; and the phosphates are 2.5 angstroms from the axis. The DNA in Pf1 is, therefore, the most extended and twisted DNA structure known. On the basis of the DNA structure and extensive additional information about the protein, a model of the virion is proposed. In the model, the DNA bases reach out, into the protein, and the lysine and arginine side chains reach in, between the DNA bases, to stabilize the paraxial phosphate charges; the conformation of the protein subunit is combination of alpha and 3(10) helices.