The three-dimensional structure of the A-capsid of herpes simplex virus type-1 has been determined to a resolution of approximately 26 A by using 400 kV spot-scan electron cryomicroscopy and computer image reconstruction techniques. The density map of the capsid has revealed several new structural details in the protein subunits of pentons, hexons, and triplexes. Our structural analysis has provided further evidence for the assignment of the four major capsid proteins to these various subunits. VP5, a 150 kDa major capsid protein that makes up both the penton and the bulk of the hexon subunits, has three domains: an upper diamond-shaped domain, a middle stem-like domain, and a lower anchoring domain. Structural differences are noticeable between the VP5 subunits in various quasi-equivalent environments. A horn-shaped mass density present at the distal end of each hexon subunit but missing from the penton subunit has been assigned to VP26, a minor 12 kDa protein. The six types of triplexes have similar, but not identical, features that include two legs and an upper domain that has a tail, which are interpreted to be formed from two copies of VP23 (36 kDa) and one copy of VP19c (57 kDa), respectively. Each triplex has two arms that interact with the adjacent VP5 subunits, and the modes of interaction vary among the quasi-equivalent triplexes. The 25 A-thick floor of the capsid is formed by the close association of the lower domains of subunits from the hexons, pentons, and triplexes. The interior of the capsid is accessible through the trans-capsomeric channels and the holes at the base of each triplex. These openings may play a role in the transport of genomic DNA and scaffolding proteins during capsid morphogenesis.