Two spontaneous mutant alleles of the L6 flax rust resistance gene, l6-X3A and l6-X117, contain the same transposable element designated dLute (defective Linum usitatissimumtransposable element). The element is 314 bp long, 70% AT-rich and, because it contains no extended open reading frame, is probably non-autonomous. It has 14 bp imperfect terminal inverted repeats related to those in the Ac family of plant transposons and, like Ac, causes 8 bp target site duplications upon insertion. Multiple copies of dLute-related sequences exist in the flax genome. Rust resistant revertants were recovered amongst the progeny of both mutants and reversion was associated with excision of dLute. Excision either restored the wild-type L6 sequence or was imprecise, leaving sequence alterations ('footprints') resulting in one to three amino acid alterations in the L6 protein. No phenotypic differences were discerned between plants containing the standard and revertant L6 alleles.