Virus particles extracted from nuclear polyhedra of Porthetria dispar L. and Aglais urticae L. by alkaline treatment may be enveloped, naked, empty, or in a variety of intermediary states. The virus envelope of P. dispar nuclear polyhedrosis virus appears to be a three-layered structure comprising an outer surface layer, in which no detailed substructure can be resolved, a layer of hexagonally packed subunits 20 nm in diameter, and a flexible membrane composed of 4-nm subunits packed hexagonally. The unenveloped virus particle consists of a core of internal component surrounded by a virus capsid (or inner membrane) composed of subunits, 3 nm in diameter, arranged in a loose type of lattice. There is a central hole or channel 10–15 nm in diameter. However, the existence of some of these structures could not be confirmed in A. urticae nuclear polyhedrosis virus particles.