Although a Chinese landrace of barley, Mokusekko 3, is completely resistant to all strains of Barley yellow mosaic virus (BaYMV) and Barley mild mosaic virus (BaMMV), and is known to have at least two resistant genes, rym1 and rym5, only rym5 has been utilized for BaYMV resistant barley breeding in Japan. In order to clarify the effect of rym1 on BaYMV and BaMMV, and to utilize the gene for resistant barley breeding, the susceptibilities of only rym1 carrying breeding lines against BaYMV and BaMMV were investigated. In the assessment of resistance to BaYMV-I, 341 F(2) populations derived from a cross between the resistant line Y4 with only rym1 and the susceptible cv Haruna Nijo shows that the segregation loosely fits a 1R:3S ratio (0.05 > P > 0.01), suggesting that the resistance is controlled by a single recessive gene, rym1. Further, none of the F(3) lines derived from the nine resistant F(2) plants showed any disease symptoms in the field infected by BaYMV-I. The same nine F(3) lines showed almost the same agronomic characters in the field infected by BaYMV-III as those in the uninfected field, apart from the symptom of showing numerous mosaics. This result indicates that the gene rym1 has an acceptable level of resistance to BaYMV-III. In the assessment of resistance to BaYMV-II, BaMMV-Ka1 and -Na1, an artificial infection method was adopted and the susceptibilities to those viruses were investigated. Although the control varieties, Ko A and Haruna Nijo, were infected with all of them, the rym1 gene carrying BC(2)F(3) lines were completely resistant to all strains. In summary, rym1 is completely resistant to BaYMV-I, -II, BaMMV-Ka1 and -Na1, and has an acceptable level of resistance to BaYMV-III. This study concludes with a discussion of the reason why the important resistance gene rym1 was eliminated along with resistant cultivars during breeding for resistance to BaYMV.