Two-hundred-and-fifty lines of groundsel (Senecio vulgaris L.) were tested for their reactions to five different isolates of the powdery mildew fungus Erysiphe fischeri Blumer. Fifty-one of the lines were also tested for their reactions to three further isolates. All the lines were tested using detached leaves but a number of them were also tested using seedlings. The seedling tests gave esentially the same results as the detached leaf tests. Over 50 % of the host lines were highly resistant (gave low infection types) to one or more of the mildew isolates and this resistance was isolate- or race-specific. Eight specific resistance factors were differentiated by the eight mildew isolates used in the tests. Some of the resistance factors appear to be present in species of Senecio other than S. vulgaris. All eight mildew isolates were complex races with virulence for all but one of the specific resistance factors, the isolates differing from each other in the possession of avirulence for a unique specific resistance factor. The interactions between the host lines and mildew isolates are indicative of a gene-for-gene interaction but the genetic analysis required to establish this was not attempted. The majority of lines which were susceptible to each isolate developed the highest or most susceptible infection type. It is considered that tolerance of the parasite may operate together with race-specific resistance in an overall survival strategy which has evolved to enable groundsel to cope with mildew attack.