Quantitative variation in susceptibility to infection by the rust Coleosporium tussilaginis, and in reduction in growth owing to the infection, were determined in 30 clonally propagated genotypes of Tussilago farfara sampled from three geographically isolated sites, Arnhem (The Netherlands), Lancaster and Sheffield (both UK). Populations, and genotypes within populations showed no significant variation in intensity of disease after artificial inoculation under ideal conditions with a fungal isolate collected from the Lancaster population. By contrast, significant differences between and within populations were observed after natural secondary infection by spores produced from the artificially inoculated plants. Genotypes differed considerably in the reduction in growth caused by the infection, but this was not correlated with the extent of symptoms shown by the plants. Low resistance was associated with high tolerance, as in the Lancaster and Sheffield populations, while high resistance was associated with low tolerance, as in the Arnhem population. The evolutionary significance of these associations is discussed.