The present paper deals with the analytical study on the genes controlling resistance to Ph.ytophthora ilefestales in potato tubers. It was shown in the former report by the author(1957) that an almost complete correlation exists between leaf and Ituber resistance, providing reliable basis for estimating leaf resistance by means of tuber trial. Strains of Phytaphthora ilufestans used were three in number; common strain H1(race 0)and two specialized strains, H2 (race4)* and H3 (race1)*. There are a number of hybrid potatoes (S. demissum x S. tuberosum) in Japan which have been bred since 1939, some of which were employed for the experiment. ' Tubers were cut in halves and a concentrated sporangial suspensio:n of the fungus was sprayed over the cut surface. There were five types of reactions classified according to the different necrotic responses (Fig. 2). As a result of inoculation test, it was revealed that all of the hybrid potatoes thnat were bred before 1956 possessed two genes, Ra and Rb for resistance singly or in combination. These two genes were recognized to follow the same pattern, independent and dominant, as was shown by many other workers. The most interesting fact is that the phenotypic expression of Rb (R4)* is of weaker type (represented by type f in Fig. 2) than that of Ra (R1)*, presenting comparatively so rich growth of aerial mycelium that sometimes one might fail to detect t:he existence of gene Rb (R4). The intervention of some modifiers in the expression of each of the above twb genes has been well surmised and these modifiers are expected to exist in S. tuberosum as well as in S. demissum.