When rice cvs. Sekiguchi-asahi and Shin 2 were inoculated with P. oryzae, orange to orange brown lesions ("Sekiguchi" lesion) were formed only on cv. Sekiguchi-asahi. On cv. Shin 2, typical necrotic blast lesion was only formed. Size of the "Sekiguchi" lesions was larger than that of commonly visible blast lesions. The "Sekiguchi" lesions rapidly enlarged, gradually merged and finally occupied the entire plant. Significant difference in lesion formation was also observed among isolates of P. oryzae used. Typical "Sekiguchi" and blast lesions were formed on the leaves by inoculation of isolates Hoku 1, Ken 54-04, 0528-2 and Naga 69-150 of P. oryzae. In cases of isolates Ina 168 and SH 85-125, only "Sekiguchi" lesions were induced. Such difference in lesion formation was already recognized at cytological observation of fungal behaviors in detached rice sheath cells. Hyphal growth of P. oryzae isolates that produced two typical lesions on cv. Sekiguchi-asahi was very good into the rice sheath cells. However, that of isolates Ina 168 and SH 85-125 that produced only "Sekiguchi" lesions was very poor. When spores of isolate SH 85-125 ("Sekiguchi" lesion inducer) were mixed with those of isolate Hoku 1("Sekiguchi" and blast lesions inducer), and the spore mixed-pastes of two isolates were punch-inoculated onto the leaves of cv. Sekiguchi-asahi, there was difference in the numbers of typical "Sekiguchi" lesions between mixed- and single-inoculation of isolates Hoku 1 and SH 85-125.