Papillae containing ultrastructurally different components were observed in mesophyll cells of Welsh onion leaves inoculated with Puccinia allii. The components of the papillae observed in the mesophyll cells before penetration by the fungus were different from those formed after penetration. The papillae found before the fungal penetration were constituted only with fibrillar components, while the papillae formed after penetration contained non-fibrillar components in addition to the fibrillar components. The non-fibrillar components were primarily found around the haustorial necks, and sometimes around the haustorial bodies. Ultrastructural analysis of mesophyll cells of the leaves (cv. Ishikura-ippon-futonegi) before the papilla formation elucidated that the plasmalemma in mesophyll cells adjacent to the haustorial mother cells was retracted from the cell walls, and tubular or vesicular structures were observed in the paramural spaces. These structures appeared to be continuous with the plasmalemma or endoplasmic reticulum in the host cytoplasm. In addition to these structures, fibrillar materials were observed in the paramural spaces. The tubular or vesicular structures may play roles in the formation of the fibrillar papillae in Welsh onion-P. allii system.