LepR3, found in the Brassica napus cv ‘Surpass 400’, provides race-specific resistance to the fungal pathogen Leptosphaeria maculans, which was overcome after great devastation in Australia in 2004. We investigated the LepR3 locus to identify the genetic basis of this resistance interaction.We employed a map-based cloning strategy, exploiting collinearity with the Arabidopsis thaliana and Brassica rapa genomes to enrich the map and locate a candidate gene. We also investigated the interaction of LepR3 with the L. maculans avirulence geneAvrLm1 using transgenics.LepR3 was found to encode a receptor-like protein (RLP). We also demonstrated that avirulence towards LepR3 is conferred byAvrLm1, which is responsible for both the Rlm1 and LepR3-dependent resistance responses in B. napus.LepR3 is the first functional B. napus disease resistance gene to be cloned. AvrLm1's interaction with two independent resistance loci,Rlm1 and LepR3, highlights the need to consider redundant phenotypes in ‘gene-for-gene’ interactions and offers an explanation as to why LepR3 was overcome so rapidly in parts of Australia.