Cotton cultivars Rowden (Gossypium hirsutum) and Seabrook Sea Island (G. barbadense) were stem-inoculated with a defoliating strain (V-44) of Verticillium dahlias and incubated in a growth chamber for three days at 25 C. Fresh stem sections unstained or stained with Sb Cl(3) reagent were examined by light or electron microscopy. Structures that appeared at the light microscopic level to be typical tyloses were found occluding vessels at two and three days after inoculation. Preliminary transmission electron microscopic studies of these inoculated cultivars generally failed to detect the typical occlusions observed by light microscopy. A revised sampling method should enable us to detect and examine the localized occlusions by electron microscopy.