The inheritance of race specific resistance to five pathotypes of the powdery mildew fungus Erysiphe fischeri Blumer was studied in different lines of Senecio vulgaris L. With two exceptions, resistance was dominant over susceptibility. The exceptions were two crosses in which resistance to pathotype 3 behaved as the recessive character. The data from these crosses suggested the involvement of dominant inhibitor genes which suppressed the activity of the resistance gene. Seven different dominant resistance genes were identified in three resistant lines, two conditioned resistance to pathotype 4, two to pathotype 10 and one each to pathotypes 3, 5 and 11. The two genes conditioning resistance to pathotype 4 were unlinked, as were the two conditioning resistance to pathotype 10. The genes for resistance to pathotypes 3, 5 and 11 were clearly linked in one line and were probably linked in the other two lines. The linkage group also appears to contain one of each of the genes for resistance to pathotype 4 and 10. The expression of the two genes conditioning resistance to 4 and 10, not in the main linkage group, appeared to be reduced in one line, probably due to the activity of modifier genes. The results are consistent with the hypothesis that race-specific resistance in S. vulgaris is controlled by a gene-for-gene interaction.