The three-dimensional structure of B capsids of the beta-herpesvirus human cytomegalovirus (HCMV) was investigated at a resolution of 3.5 nm from electron cryomicrographs by image processing and compared with the structure obtained for the alpha-herpesvirus herpes simplex virus type 1 (HSV-1). The main architectural features of the HSV-1 and HCMV capsids are similar: the T = 16 icosahedral lattice consists of 162 capsomers, composed of two distinct morphological units, 12 pentamers and 150 hexamers, with tripler structures linking adjacent capsomers at positions of local threefold symmetry. The main differences in the HSV-1 and HCMV capsids are found in the diameter of the capsids (125 and 130 nm, respectively); the hexamer spacing and relative tilt (center-to-center hexon spacing at outer edge, 17.9 and 15.8 nm, respectively); the morphology of the tips of the herons (similar in length but 33% thinner in HCRIV); and the average diameter of the scaffold (44 and 76 nm, respectively). By analogy with HSV-1, the mass on the HCMV heron tip is attributed to the smallest capsid protein (HCMV gene UL48/49). The differences in capsid structure are discussed in relation to the ability of the HCMV structure to package a genome some 60% larger than that of HSV-1.