Norwalk virus is the major cause of epidemic viral gastroenteritis of humans. Attempts to grow this virus in laboratory cell lines have been unsuccessful. However, the Norwalk virus capsid protein, when expressed in insect cells infected with a recombinant baculovirus, spontaneously assembles into virus-like particles. We have determined the 3-dimensional structure of baculovirus-expressed Norwalk virus using electron cryomicroscopy and computer image reconstruction to a resolution of approximately 22 A. These particles, having a diameter of 380 A exhibit T = 3 icosahedral symmetry. The 3-dimensional structure is composed of 90 dimers of the 58000 molecular weight (58 K) capsid protein, each of which forms an arch-like capsomere. The structure of the protein subunit is modular with three distinct domains. The distal globular domain that appears bilobed is connected to the lower shell domain by a central stem-like domain. We also have been able to grow crystals of the baculovirus-expressed Norwalk virus particles suitable for high resolution X-ray crystallography.