Abstract A pox virus was isolated from a field-collected larva of the army cutworm, Euxoa auxiliaris . Death of diseased larvae usually occurred 16–18 days after infection when from 1 to 1.5 million polyhedral inclusion bodies per milligram of larval tissue were recovered. The primary site of viral replication appeared to be the cytoplasm of fat body cells; limited infection occurred in the tracheal matrix and gut musculature. Within the fat body, oval immature virions bounded by double membranes were observed budding from viroplasms. After further maturation but before they were occluded in a protein matrix, these virions contained brick-shaped cores. Mature virions, which measured 165 × 260 nm and were loosely bounded by a wavy membrane, were occluded in polyhedra that averaged 3.7 × 4.7 μ.