K. Fukuyama, S. S. Abdel-Meguid, J. E. Johnson, and M. G. Rossmann (J. Mol. Biol. 167:873-984, 1983) reported the structure of alfalfa mosaic virus assembled from the capsid protein as a T=1 icosahedral empty particle at 4.5-A resolution. The information contained in the structure included the particle size, protein shell thickness, presence of wide holes at the icosahedral fivefold axes, and a proposal that the capsid protein adopts a beta-barrel structure. In the present work, the X-ray diffraction data of Fukuyama et al. as well as the data subsequently collected by I. Fita, Y. Hata, and M. G. Rossmann (unpublished) were reprocessed to 4.0-A resolution, and the structure was solved by molecular replacement. The current structure allowed the tracing of the polypeptide chain of the capsid protein confirming the beta-sandwich fold and provides information on intersubunit interactions in the particle. However, it was not possible to definitively assign the amino acid sequence to the side chain density at 4-A resolution. The particle structure was also determined by cryoelectron microscopy and image reconstruction methods and found to be in excellent agreement with the X-ray model.