Using leaf disc techniques, nine isolates of the mycoparasite Sphaerellopsis filum were examined for pathogenicity to Melampsora epitea, the most important species of rust in renewable energy willow plantations. Five isolates were derived from Melampsora spp. on willows and poplar, one from Puccinia coronata on couch grass, two from Phragmidium violaceum on blackberry and one from Triphragmiopsis laricinum on larch. Two inoculation experiments were carried out. In the first, S. filum and rust were applied simultaneously on to leaf discs of Salix burjatica cv. Korso. In the second, leaf discs were inoculated with rust initially, then the resulting rust uredinial pustules were inoculated with S. filum. In both experiments, all S. filum isolates from Melampsora spp., and that from P. coronata, developed pycnidia on willow rust. No pycnidia were produced from the other three isolates. Among isolates which produced pycnidia, the frequency of rust pustules colonized by S. filum and suppression of rust spore production differed markedly. In the first experiment, 55–99% pustules were colonized and rust spore production was suppressed by 64–98%. In the second, the proportions were 33–97% and 53–73%, respectively. Suppression of rust sporulation was closely correlated with the frequency of rust pustules bearing S. filum pycnidia. The results suggest that S. filum is composed of pathogenically specialized populations differing widely in their virulence.