The structure of a T = 1 icosahedral particle (where T is the triangulation number), assembled from southern bean mosaic virus coat protein fragments that lacked the amino-terminal arm, was solved by means of model building procedures with the use of 6-angstrom resolution x-ray diffraction data. The icosahedral five-, three-, and twofold contacts were found to be similar, at this resolution, to the analogous contacts (icosahedral five-, quasi-three-, and quasi-twofolds) found in the parent T = 3 southern bean mosaic virus. However, the icosahedral fivefold contacts of the T = 3 structure are the most conserved in the T = 1 capsid. These results are consistent with a mechanism in which pentameric caps of dimers are the building blocks for the assembly of T = 1 and T = 3 icosahedral viruses.