The capsid of Kaposi's sarcoma-associated herpesvirus (KSHV) was visualized at 24-A resolution by cryoelectron microscopy. Despite limited sequence similarity between corresponding capsid proteins, KSHV has the same T=16 triangulation number and much the same capsid architecture as herpes simplex virus (HSV) and cytomegalovirus (CMV). Its capsomers are hexamers and pentamers of the major capsid protein, forming a shell with a flat, close-packed, inner surface (the "floor") and chimney-like external protrusions. Overlying the floor at trigonal positions are (alpha beta(2)) heterotrimers called triplexes. The floor structure is well conserved over all three viruses, and the most variable capsid features reside on the outer surface, i.e., in the shapes of the protrusions and triplexes, in which KSHV resembles CMV and differs from HSV. Major capsid protein sequences from the three subfamilies have some similarity, which is closer between KSHV and CMV than between either virus and HSV. The triplex proteins are less highly conserved, but sequence analysis identifies relatively conserved tracts. In alphaherpesviruses, the alpha-subunit (VP19c in HSV) has a 100-residue N-terminal extension and an insertion near the C terminus. The small basic capsid protein sequences are highly divergent: whereas the HSV and CMV proteins bind only to hexons, difference mapping suggests that the KSHV protein, ORF65, binds around the tips of both hexons and pentons.