The avirulence of Erysiphe graminis f.sp. agropyri, Ak-1, on Triticum aestivum 'Norin 4' is conditioned by two major genes. When an F2 population derived from the cross between 'Norin 4' and a susceptible cultivar was inoculated with F1 hybrids between Ak-1 and E. graminis f.sp. tritici, Tk-1, resistant and susceptible seedlings segregated in either a 15:1 or a 3:1 ratio. The F1 cultures producing a 15:1 segregation and those producing a 3:1 segregation occurred in a ratio of 1:2. These results suggested that the resistance of 'Norin 4' to Ak-1 is conditioned by two major genes corresponding to the two avirulence genes. 'Norin 4' carries a resistance gene, Pm10, which operates on an F1 hybrid culture, Gw-34, but not on another F1 culture, Gw-180. Triticum compactum 'No. 44' carries another resistance gene, Pm11, which operates on Gw-180 but not on Gw-34. When these cultivars were inoculated with F2 cultures derived from the cross Gw-34 × Gw-180, avirulent and virulent cultures segregated in a 1:1 ratio. The segregation patterns on the two cultivars were independent. These results indicated that, for each of Pm10 and Pm11, there is one corresponding avirulence gene. These genes were considered to be derived from the wheatgrass mildew fungus, Ak-1. The two series of experiments strongly suggest that the forma specialis – genus specificity in the E. graminis – gramineous plant system follows the gene-for-gene theory.Key words: powdery mildew, Erysiphe graminis, wheat, wheatgrass.