The virion of human herpesvirus 6 (HHV-6) contains a very distinct tegument layer, occupying the space between the nucleocapsid and the virion envelope. Ultrastructural analyses of thymocytes infected with HHV-6 revealed the presence of intranuclear spherical compartments, approximately 1.5 microns in diameter, in which tegumentation seems to take place. These compartments, termed tegusomes, were bounded by two membranes and contained ribosomes, consistent with their derivation by cytoplasmic invagination into the nucleus. Capsids located within the nucleus outside the tegusomes were all naked, while those located in the cytoplasm were uniformly tegumented. In contrast, capsids present inside the tegusomes contains teguments of variable thicknesses. In addition, nucleocapsids were documented in the process of budding into the tegusomes. We thus suggest that the tegusomes represent a cellular site in which HHV-6 virions acquire their tegument.