Leptosphaeria maculans is the ascomycete responsible for one of the most damaging diseases of oilseed rape (Brassica napus), stem canker of crucifers. Both avirulence (AvrLm) genes in the fungus and resistance (Rlm) genes in the plant are genetically clustered. Using a map-based cloning strategy, we delineated a 238 kb region containing the AvrLm7 locus. Structural features of the region were reminiscent of those previously found on another chromosome for genomic regions encompassing AvrLm1 and AvrLm6, i.e. GC-equilibrated, gene-rich isochores alternating with AT-rich, recombinationdeficient, gene-poor isochores. These latter corresponded to mosaics of degenerated and truncated transposable elements. AvrLm7 is the only gene located within a 60 kb AT-rich isochore. It induced resistance responses in plants harbouring either Rlm7 or Rlm4, and was thus renamed AvrLm4-7. It encodes a 143-amino-acid cysteine-rich protein, predicted to be secreted, and strongly induced during early stages of plant infection. Sequencing and restriction analyses of AvrLm4–AvrLm7 or avrLm4–AvrLm7 alleles in L. maculans field isolates, and targeted point mutagenesis strongly suggested that one single base mutation, leading to the change of a glycine to an arginine residue, was responsible for the loss of AvrLm4 specificity whereas AvrLm7 recognition was unaltered.