Crossing and selfing experiments were carried out with six field collections and five isolates belonging to three formae speciales, larici-epitea typica (LET), larici-daphnoides (LD), and larici-retusae (LR), of willow rust, Melampsora epitea. European larch (Larix decidua) was inoculated with basidiospores produced on overwintered telial leaves and the resulting spermagonial lesions on larch needles subsequently paired in vitro. In two crosses between LR and LD involving a total of 439 lesion pairs, only two cultures obtained were identified as hybrids. These two cultures were non-pathogenic to the maternal host Salix burjatica cv. Korso and weakly pathogenic to the paternal host S. daphnoides cv. Meikle. No identifiable hybrids were obtained from 56 lesion pairs between LET and LD. In a cross between LET as receptor and LD as donor, one-third of the lesions formed aecia. In the reciprocal combination, however, less than 1% developed aecia. All F1 cultures between LET and LR were weakly pathogenic to the parental hosts S. viminalis cv. Mullatin and S. burjatica cv. Korso. When three of the F1 cultures were used to produce telia, only one developed mature teliospores. Subsequent selfing experiments showed that this culture was predominantly self-sterile, with only 1% of lesions producing aecia. In selfing and crossing two pathotypes within the same f. spp., 20–40% of needles produced aecia and the rate of aecial formation was similar in both directions. The results obtained suggest that M. epitea is heterothallic and the sexual compatibility is controlled by a pair of alleles at a locus. It is concluded that the three f. spp. are genetically different populations and ecologically fit new pathotypes cannot arise easily as a result of hybridization between them. Within a f. sp., however, many pathotypes exist or will occur due to exchange of genes for virulence during the sexual life-cycle.