Mating-type frequency determined for 35-687 monokaryons from 3-13 sporophores of 4 T. idahoensis field dikaryons revealed tetrapolar incompatibility in all isolates, and each sclerotial isolate yielded only 4 mating types. Matings of tester monokaryons from the 4 field dikaryons revealed 5 A alleles and 4 B alleles. Tests for virulence of 6 T. idahoensis field dikaryons on 4 winter wheats that differ in resistance revealed great differences in virulence on a given wheat. Since pathogen isolates showed no differential virulence to host cultivars, there is no evidence for a gene-for-gene host-pathogen relationship. The genetic basis of virulence was tested by forming 129 dikaryons from 28 monokaryons of field dikaryon 5999-5. Host survival following inoculation with these F1 progeny indicated that the parental dikaryon was heterozygous for several genes which determine virulence. A complete spectrum of virulence was observed in F1 and F2 dikaryons. All monokaryons were avirulent. Only a weak correlation was found between dikaryon growth rate on culture media and virulence on the winter wheat cultivar ''Nugaines''. Because all F1 and F2 dikaryons that were tested contained 4 chromosomes carrying the same complement of incompatibility genes, it is likely that incompatibility genes are not closely linked to virulence genes.