An electron microscope study has shown that the spread of Fusarium sulphureum through potato tuber tissue occurs not only intracellular but also intercellular. The spread of hyphae effected both mechanical and enzymatical. The study of cells neighbouring hyphae shows degeneration of protoplasm. Intercellular spaces of infected tissue are filled with a dark substance. It seems that these substances are fragments of cell walls which are macerated by Fusarium enzymes.