Ultrastructure of hypertrophied tissue of Puerraria thunbergiana Benth. infected with Synchytriunm minutum(Patouiellard) Gaumann was observed by means of electron microscope. The results were shown in Figs.1-14. Pathogenic fungus(PL : plasmodium) was recognized in host cell which had been enlarged by infection(Figs. 2, 3. G : giant cell). Plasmodium was observed in the cytoplasmic area of the host cell(Figs. 6, 12, 14. PL : plasmodium, HC : host cell). Plasmodium was surrounded by cell membrane of plasmodium which was adjacent to cytoplasmic matrix of host cell. Numerous nuclei, mitochondria and vesicles with electron-dense material were observed in plasmodium (Figs. 12, 13.). When the plasmodia(Figs. 2, 6.PL) became a certain stage of their life history, they were divided into uninucleate plasmodium(Figs. 12, 14.) and developed to zoosporangia(Figs. 3, 4, 5.). Zoosporangia were smooth surface, spherical and about 20μm in diamete. Plasmolysis was not detected in the cell of hypertrophied tissue. Plastids in the hypertrophied tissue cells consisted of various types, i. e. amyloplast-like(Figs. 8, 9.), proplastid-like structure and ralery, developed chloroplast(Figs. 7, 10. Fig. 11 : chloroplast in control).