Summary Analysis of the X-ray diffraction patterns from oriented specimens of narcissus mosaic virus is described. This shows that there is a marked feature in the virus particle at a radial position of about 33 Å, and it is suggested that this corresponds to the sugar-phosphate backbone of the RNA. Electron microscope studies of stained transverse sections of the virus particles are not incompatible with this interpretation. Some aspects of the possible RNA conformation in the virus particle are discussed.