The early acute response (EAR), a type of hypersensitive response, is defined by small chlorotic spots at the base of the youngest leaf of sorghum (Sorghum bicolor L. Moench) cultivar HOK, and usually appears within five days after inoculation with maize dwarf mosaic virus strain A (MDMV-A). These chlorotic spots become necrotic one to two days later and the leaf tissues are rapidly killed. In leaf tissues showing EAR, plasmodesmal fields contained many modified plasmodesmata of various sizes and structures within thickened cell walls. The membranous vesicles and tubules, derived from the extended terminal structures of modified plasmodesmata, were blocked by callose-like deposits in the area between the cell wall and plasmalemma. Also observed were two opposite-directed channels united via a central cavity at the middle lamella of the cell wall, one end of which was connected to the plasmalemma, but the other end sealed off to form a bulbous extension. The localized structure, an extraprotoplasmic saccontaining aggregates of elongated virus-like particles associated with the modified plasmodesmata, was located between the plasmalemma and the cell wall. The sac was bound by membranes, and appeared to be sealed and completely excluded from the protoplasm. Extraprotoplasmic sacs appeared to be derived from the terminal extensioj of modified plasmodesmata, and these modification seem to be related to restriction of the viral spread.