Herpesviruses form an extensive family of DNA viruses, in which three subfamilies called alpha-, beta- and gammaherpesviruses, respectively, are distinguished on the basis of biological properties. In terms of overall structure, all herpesviruses conform to a common plan: the virion consists of a thick-walled capsid, ∼ 1250 Å in diameter, containing the genome, surrounded by a complex layer of proteins called the tegument (a feature unique to herpesviruses), and a lipoprotein envelope. At a more detailed level, it has not been clear whether the three subfamilies are also differentiated structurally. The molecular architecture of alphaherpesvirus capsids has been studied by cryoelectron microscopy of herpes simplex virus 1, revealing its icosahedral (T=16) surface lattice and the locations of the four abundant shell proteins. To ascertain how closely these features are emulated in a betaherpesvirus, we have studied capsids of simian cytomegalovirus (SCMV) by similar methods. By comparing the structures of nuclear capsids and cytoplasmic capsids (which have bound some tegument proteins),