The most common and eVective way to control phoma stem canker (blackleg) caused by Leptosphaeria maculans in oilseed rape (Brassica napus) is by breeding resistant cultivars. SpeciWc resistance genes have been identiWed in B. napus and related species but in some B. napus cultivars resistance is polygenic [mediated by quantitative trait loci (QTL)], postulated to be race non-speciWc and durable. The genetic basis of quantitative resistance in the French winter oilseed rape ‘Darmor’, which was derived from ‘Jet Neuf’, was previously examined in two genetic backgrounds. Stable QTL involved in blackleg resistance across year and genetic backgrounds were identi- Wed. In this study, near isogenic lines (NILs) were produced in the susceptible background ‘Yudal’ for four of these QTL using marker-assisted selection. Various strategies were used to develop new molecular markers, which were mapped in these QTL regions. These were used to characterize the length and homozygosity of the ‘Darmor-bzh’ introgressed segment in the NILs. Individuals from each NIL were evaluated in blackleg disease Weld trials and assessed for their level of stem canker in comparison to the recurrent line ‘Yudal’. The eVect of QTL LmA2 was clearly validated and to a lesser extent, QTL LmA9 also showed an eVect on the disease level. This work provides valuable material that can be used to study the mode of action of genetic factors involved in L. maculans quantitative resistance.