The seeds of two varieties Shfih6 and BR-No. 1 were irradiated with T-rays (60Co) of 15KR, and the leaf-blast resistance to physiological races N-1 and C-1 was analyzed genetically in the X5 gene-ration. Only susceptible mutations were obtained in both varieties. No relation could be fcund between the mutation frequency and X1-panicle sterility. The mutation spectrum was very different between the varieties Shuho and BR-No. 1. The mutants derived from Shuho which revealed only the S-type symptom showed a wide range of variation in the number of S-type lesions. These lesions were assumed to be caused by polygenic and/or major-genic mutations. As for BR-No. 1, however, no polygenic mutations could be detected and all the mutants obtained showed only S-type lesions.. regarded as the result of the mutation of one or a few major genes. Experimental results suggest that the resistance of BR-No. 1 to race N-1 was controlled by one gene and the resistance to race C-1 was controlled by the simultaneous effect of two genes or the effect of epistasis of the two alleles originated in Shuho and Hokushin-Asahi. In spite of the S-type symptom, Shuho has a very stable field-resistance. The stability of this variety was discussed from the genetic point of view.