AbstractElectron microscope observations ofCeratocystis ulmi‐induced tylosis development in 3 species of elm were made during the first 2 weeks after artificial inoculation. Tylosis forming from both ray and vasicentric parenchyma occluded vessels adjacent to those containing the fungus conidia within one day after inoculation. The expanding protoplast forced the intact vessel‐parenchyma “pit membrane” and protective layer through the pit into the vessel lumen. Cell organization during this expansion is discussed. The observations on tylosis development for the 3 elms used were id