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Resolving the nonicosahedral components in large icosahedral viruses remains a technical challenge in structural virology. We have used the emerging technique of Zernike phase-contrast electron cryomicroscopy to enhance the image contrast of ice-embedded herpes simplex virus type 1 capsids. Image reconstruction enabled us to retrieve the structure of the unique portal vertex in the context of the icosahedral capsid and, for the first time, show the subunit organization of a portal in a virus infecting eukaryotes. Our map unequivocally resolves the 12-subunit portal situated beneath one of the pentameric vertices, thus removing uncertainty over the location and stoichiometry of the herpesvirus portal. Herpes simplex virus type 1 (HSV-1) is an enveloped mam-malian virus approximately 200 nm in diameter, consisting of a 125-nm icosahedral capsid surrounded by tegument and enve-lope (17). The HSV-1 genome is packaged into the capsi