The three-dimensional structures of the procapsid and of the mature capsid of bacteriophage lambda were determined to a resolution of approximately 3.4 nm by cryo-electron microscopy and image processing. The mature lambda capsid contains two major proteins, gpE and gpD, arranged on a T = 7 lattice, with gpE arranged as hexamers and pentamers and gpD arranged as trimers. The hexamers and pentamers in the virion display a cartwheel-like structure, with skewed spokes (or arms) radiating out from a central hexameric hub. The thimble-shaped gpD trimers are superimposed on the trivalent interaction point of these arms. A reconstruction of a lambda D- mutant capsid to lower resolution shows no trace of these trimers, thus revealing the interactions of the underlying arms. The procapsid has elongated, irregularly shaped hexamers with gpE subunits set perpendicularly to the capsid surface.