The pathogenic variability of the barley [Hordeum vulgare] scald fungus, R. secalis, in California [USA] was examined. One hundred seventy-five isolates of the fungus, obtained from naturally infected barley in 23 counties, were differentiated into 75 pathogenic races on 14 barley cultivars representing many of the currently known genes for specific resistance to scald. The 75 races encompassed a wide spectrum of pathogenicity and included a race pathogenic to none of the differentials as well as a race pathogenic to all of them. Four races, which accounted for 37% of the isolates, included 2 races from each end of the race spectrum; i.e., 2 races pathogenic to relatively few of the differentials and 2 races pathogenic to most of the differentials. The group of races pathogenic to most of the differentials was concentrated in the southern San Joaquin Valley, the major barley-producing area of the state. The differential reactions of the cultivars revealed the relative effectiveness of the various sources of resistance to the California population of the fungus. The most effective cultivars, ''C.I. 5831'', ''Hudson'' and ''Turk'', were susceptible to 26, 34 and 36%, respectively, of the population. The disease reactions of the cultivars in the host range revealed differences in the resistance of cultivars previously thought to carry identical resistance genes. The cultivars involved either had additional genes for resistance and susceptibility or the genes previously thought to be identical were different.