The objective of these studies was to find out a laboratory testing method for blast field resistance in rice. Rice plants at 5 to 6 leaf-stage were inoculated with the seven differential blast strains of Pyricularia oryzae which have been extensively used in Japan and some mutant strains under greenhouse conditions. The number of conidia of Pyricularia oryzae developed on susceptible lesions was compared each other and its differences were expressed as sporulatioil of the fungus. In addition, varietal difference in sporulating results could be summarized as follows: 1. It was revealed that there was a significant difference in sporulation of the seven blast fungus strains and the used some mutant strains. 2. A positive correlation was detected between sporulation and two factors. expansion rate of lesions and aggressiveness on the host plants. On the other hand, a negative correlation was found between sporulating ability and number of conidia developed on the oat-meal-sucrose-agar while there was no relationship between length of lesion and number of the conidia developed on the barley-grain medium. 3. The maximum sporulation appeared on the second day after appearance of susceptible lesions of the plant. The proper incubation period :for development of susceptible lesions varied from one experiment to another, depending upon the nutritional conditions of the host plants. 4. In some experiments, a negative density effect of sporulation of the fungus was shown as number of lesions was getting increased. 5. A more clear inter-strain difference in sporulation was oberserved as compared with that in aggressiveness, indicating that the latter was more greatly affected by environmental conditions rather than the former. Therefore, sporulation of the fungus might be more useful for field resistance test of rice varieties to blast disaese than the aggressiveness test, provided that a consistent relationship between these two factors exists. 6. Varietal difference in resistance to sporulation of blast fungus in rice was demonstrated in several experiments but no such difference in development of lesions was detected. However, it appeared that the field resistance seemed to be more closely correlated with resistance to lesion development than with that to sporulation of the fungus. 7. Less sporulation of the mutant strains in susceptible rice varieties as compared with their original strains suggested that there is a possibility of the stabilizing selection among fungus strains.