Loose smut, caused by the fungus Ustilago tritici (Pers.) Rostr., is a common disease throughout the wheat-growing regions of the world. The development of resistant cultivars is the most effective and economical strategy for managing this disease. Sonop (TD-14) is a South African spring wheat variety that carries a high level of resistance toloose smut. This project was undertaken to study the genetics of loose smut resistance in TD-14. A doubled haploid (DH) population of 168 lines derived from the cross of Diamont/TD-14 where Diamont is susceptible to U. tritici was studied. The parents and progenies were inoculated with U. tritici races T2, T9 and T39individually in growth facilities at Swift Current and Winnipeg, Canada and loose smut incidence (%) was assessed. Genotyping of the DH population and parents using an Infinium iSelect 90K single nucleotide polymorphism (SNP) array identified 11,679 polymorphic SNPs. A preliminary linkage map of the population consisted of 33 linkage groups and spanned 3,324 cM. The frequency distributions of loose smut incidence among Diamont/TD-14 DH lines showed a bi-modal distribution for race T2, suggesting a single gene controlled this resistance. Frequency distributions for T9 and T39 were continuous, which prevented classifying DH lines into discrete resistant and susceptible classes. Interval mapping (IM) and inclusive composite interval mapping (ICIM) analyses identified four quantitative trait loci (QTL) controlling loose smut resistance in the Diamont/TD14 DH population on chromosomes 7B, 3B, 5B, and 5D with TD-14contributing the resistant alleles. The location of T2 resistance mapped as a qualitative trait was close to the major QTL on chromosome 7B suggesting this QTL was responsible for the bi-modal distribution of this trait. Another major QTL located on chromosome 5B was effective against all the three races and it explained up to 68% of the phenotypic variation. This study provides insight into the genetic basis of loose smut resistance in spring wheat variety TD-14and the QTL on chromosomes 7B and 5B may be effective genetic sources for breeding loose smut resistance in spring wheat. 019109