Cells in the interior of susceptible oat roots treated with the disease‐inducing agent victorin exhibit many of the ultrastructural features which characterize the epidermal or outermost root cap cells of untreated roots. An increase in electron density of cell walls fixed in permanganate is the first effect of victorin seen in the root interior. Other early victorin‐induced changes are formation of enlarged, densely stained vesicles by the Golgi apparatus and organization of the endoplasmic reticulum into roughly parallel profiles. All of these features are characteristic of untreated epidermal cells. Victorin also induces the formation of large numbers of lomasome‐like wall lesions and causes a marked increase in the number of nearly spherical, membrane‐bounded structures tentatively identified as spherosomes. Similar lomasome‐ and spherosome‐like structures are much more abundant in the outermost cells of the root cap than in other regions of untreated roots. This suggests that these structures may be characteristic of cells destined to undergo disintegration. Victorin‐induced lesions appear to arise within the cell wall as the result of an activation of wall‐degrading enzymes. An early change which makes the unit structure of the plasma membrane visible over extended areas may account for victorin‐induced changes in permeability. Disrupted plasma membranes and swollen mitochondria are found only in cells heavily damaged by victorin. Many of the effects of victorin resemble those of calcium deficiency and calcium is known to suppress victorin‐induced disease symptoms. This suggests that calcium nutrition may play a role in the pathological changes induced by victorin.