Crystals of Cowpea mosaic virus displaying a high degree of order have been grown in two different forms. Hexagonal crystals, space group P6(1)22 or P6(5)22 with a = 450 A and c = 1038 A containing six virus particles per cell, have been grown in sodium citrate buffer at pH 4.9. Each particle has four nearest neighbors at 284 A. Cubic crystals, with space group I23 and a = 308 A , containing two particles per cell were grown from ammonium sulfate at pH 4.9. In the cubic crystals, the tetrahedral subgroup of the icosahedral particle symmetry is expressed as part of the crystal lattice, with each particle occupying tetrahedral crystallographic positions. Contacts of 268 A occur between particles in contact along threefold icosahedral symmetry axes.