|| A total of 162 strains of wild barley, Hordeum spontaneum C. Koch originating from Iran, Iraq, Turkey and Central Asia, were tested for resistance to powdery mildew. Then, the variation of resistance was compared with that of 145 local varieties of cultivated barley (Hordeum vulgare L.) originating from the same region of the wild barley collection. Ten different isolates of the parasite with Japanese origin were separately inoculated onto the first leaves of the host plants. The infection types were classified into the following: i, immunelike; R, highly resistant; M, moderately resistant; and S, highly susceptible. Resistant strains with i, R or M infection type were more frequent among wild barleys as compared with the cultivated forms. It is noteworthy that among these three resistant reactions,the M type was most frequent in the wild barley. To compare the degree of resistance to a total of 10 isolates, the resistance score was calculated in each of the wild and cultivated strains as the following: Scores 1,2,3 and 4 were given to the infection types of i, R, M and S, respectively, and the mean score for 10 isolates was calculated. Wild barley showed significantly low resistance scores as compared with those of cultivated barley. This was also confirmed by the cluster analysis; the cluster with more resistance to 10 isolates consisted of many strains of wild barley. Next, the resistance of wild barley was characterized by their broader effective ranges to different isolates. According to the x2 test for independence of reactions to two different isolates, the resistant factor(s) involved in wild barley was confirmed to be rather non-specific to the parasite. It was concluded that H. spontaneum may be useful genetic resources for the breeding of resistance to powdery mildew as well as local varieties.