Successive microscopical observations were made on living sections stripped from the midrib epidermis of highly resistant potato variety 41089-8 (Solanum demissum × S. tuberosum) infected with Phytophthora infestans. These sections were placed between slide glass and cover glass supported with small glass rods, and mounted in water. Vital staining method with neutral red, and plasmolyzing method with sucrose solution were used to ascertain the moment when the host cell and intracellular hyphae die. If the sections are stripped with razor, when the invaded cells are still normal, or the contents already discolor but the granules are still in Brownian motion (4∼8hrs. after inoculation), the invaded cells discolor lightly in the course of successive observations, but do not become brown. The most of intracellular hyphae in these sections survive for a long time (average about 30∼50hrs., max. about 90hrs. after inoculation) and develop into the surrounding tissues. In the sections stripped when the invaded cells are still yellow or pale brown but granules have already stopped their Brownian movements, approximately a half of the intracellular hyphae have survived and subsist longer (average about 30hrs., max. about 50hrs. after inoculation) than those in intact tissue, but they are unable to grow further in the cell. In the case when intracellular hyphae are in touch with the cell wall, they grow occasionally toward the neighbouring cell in which the granules are still in active Brownian movements. Accordingly it is assumed that the growth of these hyphae has been inhibited mechanically by the gelation of host cell contents. In these sections, no more browning of the invaded cell are observed. It seems that, when the sections were stripped from the materials in which the invaded cells had already turned blackish brown, no intracellular hyphae were alive. In these sections, it was ascertained that the tissues around the injured cells survived for 1∼2 days after sectioning.Following experiments were carried out to learn the reason why the browning process of invaded cell can not continue further in these living sections. The sections, invaded cells of which were in an early stage of alterations, were placed on pure agar in petri dish, and exposed to the air (oxygen) for 22 to 23hrs. The invaded cells in these sections showed discoloration, but could not accomplish the normal browning process, and the intracellular hyphae survived for a long time. These experiments show that inhibition of the browning process in these living sections is not necessarily due to the lack of oxygen, but to the loss of the active vital function.Other experiments showed that increase of phenol compounds and activation in N-metabolism were seen in the tissues neighbouring the cells infected with phytophthora infestans, and these compounds were deposited continuously on the latter cells.It seems that in the course of browning process, the continuous deposition of these compounds may result in the gelation of the cell contents, the sealing up of hyphae and finally the death of them. It may therefore be said that the “active vital function” described above is related to these processes.