The Emperor Gum moth ( Antheraea eucalypti Scott) is susceptible to a cytoplasmic polyhedrosis of the mid-gut. The disease was observed in larvae reared in the laboratory and has not been found in those from the field. The infected larvae first become lethargic and cease feeding. Later the cuticle changes colour and becomes soft. The mid-guts of dead larvae are white, very thin, and about one-third their normal length and the gut cells are packed with polyhedra. In electron micrographs the polyhedra show cubic symmetry with 42 Å spacings between the centres of the protein molecules. The virus particles are spherical and approximate 370 Å in diameter. The virus is extremely pathogenic to A. eucalypti larvae. Of six other species of Lepidoptera which were fed the polyhedra, five developed symptoms of a cytoplasmic polyhedrosis.