In the small regular virus particles, the (infective) nucleic acid is protected in a protein container built from many identical protein molecules. Early x-ray diffraction work showed that these subunit molecules are arranged with helical symmetry to form rod-shaped particles, or with icosahedral symmetry to form isometric particles. Current work on the rod-shaped, tobacco mosaic virus is directed towards e 10 Å resolution electron density map of the virus; a parallel study on crystals of the protein subunits has so far yielded a 15 Å map. The structures of two isometric viruses have been obtained to a resolution of 25 Å; both of these have extra, non-icosahedral, local symmetry of a type which had been predicted on theoretical grounds.