Abstract Polyhedral particles in crude extracts and partially purified preparations from leaf gall tissue of Fiji disease virus (FDV)-infected sugarcane have been examined in an electron microscope after negative staining or freeze-drying and shadowing. The FDV particle is a double-shelled, icosahedral structure consisting of a stable core surrounded by an unstable outer shell. The intact virus particle in freeze-dried and shadowed preparations is about 67 nm in diameter with projections (A spikes), about 14 nm wide and 8 nm long, at the 12 vertices of the icosahedron. The A spikes are attached to the projections (B spikes) of the 54-nm core which are about 19 nm wide and 9 nm long. The resolution of the FDV-particle substructure in negatively stained preparations was not sufficient to determine the size and arrangement of the capsomeres. The morphology of FDV was compared to that of reovirus prepared for electron microscopy by similar techniques; although the two viruses possess similar general structure and complexity, some significant differences were readily recognized. It is concluded that the FDV particle is remarkably similar to that of maize rough dwarf virus, confirming the suggestion that the two viruses are closely related.