Ultra-thin sections of the hypertrophied tissues of maize infected by Ustilago maydis (de Condole) Corda were studied under the electron-microscope. Fungal hyphae were recognized in inter-cellullar spaces middle lamella and inside of the cell of the suscept tissue. Although the tips of hyphae were recognized in the cytoplasmic layer of the suscept cell, the hyphae(probably haustorium) were always surrounded and separated from the suscept cytoplasm by a membrane. Hypertrophied cell of the suscept had a large vacuole and thin peripheral layer of cytoplasm. Amyloplasts and chloroplasts were not found in the cytoplasm. Osmiophilic granules were observed but the developed lamella structures were not observed in the plastids.