Fourteen of the known genes conferring resistance to rust in flax occur in the L group, and recombinational analysis has been used to study their fine structure. Three important features were observed. (a) Similar to the findings of Shepherd and Mayo, only susceptible recombinants were detected among the testcross progeny of 11 of the 15 heterozygotes involving pairs of L genes. Some of these recombinants showed variation in the degree of their susceptibility and appeared to be unstable in nature. (b) A new class of recombinants exhibiting a modified type of resistance was recovered. They occurred rarely but consistently, with frequencies similar to that of susceptible recombinants. (c) Rare resistant plants occurred among the progeny of susceptible recombinants. In each case, the specificity of the resistant plant corresponded to only one of the parental types. The relative roles of seed contamination, mutation, recombination and the transposition of genetic elements are discussed to account for these features.