Using disomic chromosome substitution lines based on the susceptible wheat cultivar 'Chinese Spring', loose smut resistance of wheat cultivars 'Hope' and 'Thatcher' was shown to be conferred in each case by a single dominant major gene carried on chromosome 7 A ('Hope') or 7 B ('Thatcher'). Partial resistance was determined by genes on an additional eight 'Hope' or seven 'Thatcher' chromosomes, and similarities were evident between the partial resistance genotypes of'Hope' and 'Thatcher'. 'Chinese Spring' exhibited a mean infection value of approximately 50%, indicating a significant level of partial resistance, which was found to be due, in part, to genes on the homoeologous chromosome arms 1 As, 1 Es and 1 Ds, and to cytoplasmic genes. Substitution of the 'Chinese Spring' nucleus into the cytoplasm of Aegilops squarrosa, Ae. variabilis or Ae. mutica resulted in increased susceptibility to Ustilago tritici. Several alloplasmic lines of the resistant wheat cultivars 'Selkirk' and 'Chris' exhibited race-specific susceptibility to U. tritici.