Technical Abstract: ABSTRACT A toxin, designated as SnTox1, was partially purified from culture filtrates of isolate Sn2000 of Stagonospora nodorum, the causal agent of wheat leaf and glume blotch. The toxin showed selective action on several different wheat genotypes indicating that it is a host-selective toxin (HST). The synthetic hexaploid wheat 'W-7984' and hard red spring wheat 'Opata 85', the parents of the ITMI mapping population, were found to be sensitive and insensitive respectively, to SnTox1. The ITMI mapping population was evaluated for toxin reaction and used to map the gene conditioning sensitivity. This gene, designated as Snn1, was mapped to the distal end of the short arm of 1B. The wheat cultivar 'Chinese Spring' (CS) and all CS nullisomic-tetrasomic (NT) lines were sensitive to the toxin, with the exception of N1BT1D. Insensitivity was also observed when the 1B chromosome of CS was substituted with the 1B chromosome of an insensitive accession of Triticum dicoccoides. In addition, a series of 1BS chromosome deletion lines were used to physically localize the sensitivity gene. Physical mapping indicated that Snn1 lies within a major gene-rich region on 1BS. This is the first report identifying a putative proteinaceous HST from S. nodorum and the chromosomal location of a host gene conferring sensitivity.