Narcissus mosaic virus is a Potexvirus, a member of the Flexiviridae family of filamentous plant viruses. Fiber diffraction patterns from oriented sols of narcissus mosaic virus have been used to determine the symmetry and structural parameters of the viral helix. The virions have a radius of 55 ± 5 Å. The viral helix has a pitch of 34.45 ± 0.5 Å, with 7.8 subunits per turn of the helix. We conclude that all members of the Potexvirus genus have close to 8 subunits per helical turn.