Genetic diversity among 51 isolates of the mycoparasite Sphaerellopsis filum from Melampsora rusts on willow and poplar was examined using AFLP. Genetic variation was relatively low among the isolates (Nei & Li's similarities > or =90). Genetic diversity calculated using Shannon index was 0.119 at Loughgall, Northern Ireland, 0.109 at Markington, northern England, 0.039 at Craibstone, Scotland, and 0.015 at Long Ashton, southwest England. At Long Ashton, 14 out of 16 isolates shared the same AFLP bands. Two genotypes were found at both Markington and Loughgall. The low genetic diversity and a high rate of clonality suggested that asexual reproduction plays a major role in S. filum epidemics. Sequence information was also obtained from the ITS-5.8S region of the ribosomal DNA from the S. filum isolates derived from willow and poplar rusts and six isolates derived from other sources. ITS sequences were identical among all the 51 isolates from willow and poplar rusts. ITS analysis placed S. filum isolates from Melampsora spp. on willow and poplar, Puccinia coronata on grass and Melampsora sp. on Euphorbia sp. into one clade and the isolates from blackberry rust Phragmidium violaceum and larch rust Triphragmiopsis laricinum into another. Nucleotide sequence differences between the two groups were 8.4-10.4 %. The ITS-5.8S sequences obtained in this study were compared with those deposited in the GenBank database.