ABSTRACT The fungus Pyrenophora tritici-repentis produces a toxin (Ptr ToxA) that causes rapid cell necrosis in sensitive wheat genotypes. A single recessive gene (tsn1) on chromosome 5BL in common wheat confers insensitivity to this toxin. Our objectives were to analyze the allelic relationships of genotypes that have shown insensitivity to a P. tritici-repentis necrosis-inducing toxin, map the gene for insensitivity to the necrosis-inducing factor produced by P. tritici-repentis in a durum wheat population, and determine the reaction to P. tritici-repentis of aneuploid genotypes that do not contain the gene. Greenhouse-grown plants of seven populations from crosses of insensitive genotypes; an F(2) population of durum wheat; and 'Chinese Spring' aneuploid, substitution, and deletion lines were infiltrated with Ptr ToxA. All crosses involving insensitive genotypes failed to produce sensitive progeny, indicating that the same gene is present in these genotypes. The gene for insensitivity in the durum population was mapped to the same region on 5BL as in common wheat using restriction fragment length polymorphism markers. 'Chinese Spring', its homoeologous group 5 nullisomic-tetrasomic stocks, and 5BL deletion lines were insensitive to the toxin. Substitution of a 5B chromosome from sensitive genotypes into 'Chinese Spring' resulted in sensitivity. Therefore, insensitivity is not conferred by a gene product per se, but rather conferred by absence of a gene for sensitivity.