Hyphal growth and host responses were investigated in rice leaf-sheaths inoculated with Pyricularia oryzae and other Pyricularia spp. When the sheaths of the nine rice cultivars, having different genes of resistance to P. oryzae, were inoculated with the incompatible race of rice blast fungi, there were large differences in hyphal growth among the cultivars. In the cvs. Kanto 51 and Toride 1, the growth of most intercellular hyphae of the incompatible races was very poor in the values of both the average and highest indices; all invading hyphae were located in the first invaded cells. In the cvs. Ishikari-shiroke and Yashiro-mochi, however, most invading hyphae of the incompatible races developed well into the sheath cells; fungal behaviors and morphological changes of host cells showed almost same tendency as those in the compatible race-cultivar interactions. When non-detached sheaths of those cultivars were inoculated with the incompatible races of P. oryzae, blast lesions similar to susceptible lesion were formed on the surface of leaf-sheath 10 days after inoculation. On the other hand, in case of non-host interactions with non-pathogenic Pyricularia spp., hyphal growth was very poor in all combinations.