A culture of race 218, M. lini was selfed and the progeny were tested for virulence on flax lines each of which contained a pair of genes conditioning rust resistance. Race 218 was crossed with race 22, but only a limited number of F1 cultures were derived. Race 218 and all of the S1 [1st generation self] cultures resulting from selfing race 218 were virulent on near-isogenic flax lines containing L5, L6, L7, L9, L11, M1, M2, M4 and P1 indicating that the genes for virulence were homozygous. Race 218 also was virulent on the line with L10. Segregation for virulence on the line with L10 may have been due to a temperature-sensitive factor. Race 218 and all the S1 progenies tested were avirulent on lines containing L2, N, P4 and Kugine, indicating that race 218 is homozygous avirulent on lines with these resistance genes. Segregation ratios fit theoretical monogenic recessive ratios on isogenic lines possessing host genes K, L1, L3, L4, L8, M, M3, M5, M6, N1, P, P2 and P3. Virulence on the line with N2 probably was conditioned by a single recessive gene since there were only a few more cultures in the virulent category than would fit expected ratios. Two independently inherited recessive genes apparently conditioned virulence on the line with L. Chi-square tests indicated that virulence on several lines with alleles at the L locus and the P locus was linked. Usually virulence on most other lines with allels at different loci was inherited independently; the few exceptions included virulence on lines with L8 and M which were apparently closely linked. These linkage patterns indicate that a deletion of a small segment carrying A1 8 and AM could cause virulence on flax lines carrying the corresponding resistance genes.