Porcine cytomegalovirus was examined by electron microscopy in the nasal mucosa of piglets from a natural epizootic of the disease with the object to determine the localization of nucleocapsids arranged in crystalline arrays in the cell and to investigate the structure of most common viral forms. The study revealed the following. 1. The morphogenesis of porcine cytomegalovirus is marked by intranuclear formation of nucleocapsids arranged in crystalline arrays. Large quantities of crystals found in the cytoplasm were apparently extruded from the nucleus at late stages of infection; 2. nucleocapsids in the cytoplasm were coated with an electron dense amorphous material. This coat was also seen at the surface of capsids in cytoplasmic and extracellular enveloped particles, but was absent from the surface of capsids located in the nucleus; 3. the majority of enveloped virus particles showed an electron translucent zone separating the capsid from the envelope, and 4. the outer envelope of cytoplasmic and extracellular particles had projections and showed a clear unit membrane structure, whereas the outer envelope of intranuclear particles and those located in the perinuclear cisterna had the appearance of an electron dense single membrane.