Ultrathin sections of mesophyll cells of the rice(Oryza sativa L. cv. Kibiyoshi) infected with rice necrosis mosaic virus were examined with an electron microscope. Two types of cytoplasmic inclusion were observed in the cells. One was similar to the cylindrical inclusion which was observed as pinwheels or bundles mainly produced by filamentous viruses. The other was the membranous inclusion(MI) which seemed to be composed of the tubules closely assembled. Cross sections of MI showed elliptic or circular shape, about 4μm in diameter. The membranes composed of the tubules communicated with each other and associated with endoplasmic reticula at the terminal sites. The tubules were disposed in a hexagonal pattern like honeycomb and each tubule was surrounded by 6 spots which appeared like particles, about 10-12nm in diameter. The spots in cross view turned into filaments by tilting the sections at an angle of 40°. Therefore, it seems that these spots are cross view of the filaments running in parallel with the tubules but not the virus particles. MI observed in this experiment seems to have the most complex structure among the inclusions related to plant viruses reported hitherto.