Crystals of Mengo virions have been grown reproducibly and analyzed by X-ray diffraction. These crystals diffract to a resolution of 7.0 A. The unit cell exhibits cubic symmetry with a = 422 A. The space group is P23, with four virus particles situated on crystallographic threefold axes. Picornavirions from three of the four recognized genera (Study Group on Picornaviridae, Intervirology 10, 165-180, 1978) have now been examined at low resolution by X-ray diffraction: poliovirus type 1 (J. T. Finch and A. Klug, Nature (London) 183, 1709-1714, 1959; J. M. Hogle, J. Mol. Biol. 160, 663-668, 1982); human rhinovirus 14 (J. W. Erickson, E. A. Frankenberger, M. G. Rossmann, G. S. Fout, K. C. Medappa, and R. R. Rueckert, Proc. Natl. Acad. Sci. USA 80, 931-934, 1983); and Mengo virus.