Bacteriophage T7 is a double-stranded DNA bacteriophage that has attracted particular interest in studies of gene expression and regulation and of morphogenesis, as well as in biotechnological applications of expression vectors and phage display. We report here studies of T7 capsid assembly by cryoelectron microscopy and image analysis. T7 follows the canonical pathway of first forming a procapsid that converts into the mature capsid, but with some novel variations. The procapsid is a round particle with an icosahedral triangulation number of 7 levo, composed of regular pentamers and elongated hexamers. A singular vertex in the procapsid is occupied by the connector/portal protein, which forms 12-fold and 13-fold rings when overexpressed, of which the 12-mer appears to be the assembly-competent form. This vertex is the site of two symmetry mismatches: between the connector and the surrounding five gp 10 hexamers; and between the connector and the 8-fold cylindrical core mounted on its inner surface. The scaffolding protein, gp9, which is required for assembly, forms nubbin-like protrusions underlying the hexamers but not the pentamers, with no contacts between neighboring gp9 monomers. We propose that gp9 facilitates assembly by binding to gp10 hexamers, locking them into a morphogenically correct conformation. gp9 is expelled as the procapsid matures into the larger, thinner walled, polyhedral capsid. Several lines of evidence implicate the connector vertex as the site at which the maturation transformation is initiated: in vivo, maturation appears to be triggered by DNA packaging whereby the signal may involve interaction of the connector with DNA. In the mature T7 head, the DNA is organized as a tightly wound coaxial spool, with the DNA coiled around the core in at least four and perhaps as many as six concentric shells.