In the present study a series of experiments were carried out to measure the time required for young midrib cell of resistant and susceptible potato varieties to become brown when infected with Phytophthora infestans. The varieties used in the experiments are 41089-8 (Solanum demissum×S. tuberosum) as the resistant one, and Hokkai No. 9 (S. tuberosum) as the susceptible one. As has been shown in the former reports9, 10) when microscopical observation is successively made on the same living section, the degeneration process seems to be retarded by the injury caused in the act of handling.Accordingly in the present study microscopical observations were made always on fresh and living sections stripped by razor at a definite interval of time 10-20min. (in the case of 41089-8) or 1-several hours (in the case of Hokkai No. 9) after inoculation. The observations were finished in short time. Inoculation experiments were repeated 14 times in the case of 41089-8, and 7 times in the case of Hokkai No. 9.Seven phases were distinguished in the process of infection of the host cells by P. infestans. The number of cells belonging to each phase was divided by total number of cells observed and expressed in percentage in figs. 1 and 2. By plotting the values thus obtained as ordinates against time as abscissas, the curves shown in figs. 1 and 2 were obtained. From these curves, the time required for completion of each phase of the degeneration process of cells infected by P. infestans may be deduced (Table 1).About 12/3∼2 hrs. after inoculation, penetration was observed on midrib epidermis of the resistant variety, showing no difference from that in the susceptible variety. After the fungus penetrated into the cell and formed the primary intra-cellular mycelium, remarkable differences in the degeneration process of cells began to be observed between the two varieties.In the midrib cells of the highly resistant variety“41089-8”, within 10min. to 1 hour after completion of penetration, the first sign of injury appeared; the granules in Brownian motion appeared around the infected loci. About 10 minutes to 1 hour after the appearance of such granules, the contents in the infected cell began to discolor. Within about 10 minutes to 30 minutes after the beginning of discoloration, the granules stop their movement, and the cell content seems to gelatinize. In midrib cells of the susceptible variety, on the contrary, the granules in Brownian motion do not appear in the infected cell until 2 to 8 hours after the completion of penetration. Within about 7 to 10 hours after the appearance of such granules, discoloration of the content in infected cell does not take place. The granules stopped their movement 1 to 11/2 hours after discoloration of cell contents.In such process of reaction of the resistant and susceptible varieties, the most quick degeneration was observed in the basal tissue of midrib of unfording leaflets. The time required for degeneration seems to be greater in the tissues of comparatively older leaflets than in younger ones.Within about 10 hours after gelatinization of cell contents in infected cells, the brown color of that cell becomes deeper and finally blackish brown. Thereafter, however, it seems that further deepening in color in infected cells does not take place. Such deepening in color suggests that the deposition of polyphenol and other compounds in the browned infected cell continues for a comparatively longer period after the collapse of that cell.