Race 370, M. lini, was selfed and the progeny were tested for virulence on flax cultivars each of which contained a single gene pair for rust resistance. All cultures tested were virulent on flax cultivars with resistance genes L9, M, M1, M2, M4, N1 and P and avirulent on flax cultivars possessing the K, L2, L3, L5, L6, L7, L11, M3, M5, M6, N, N2, P1, P2, P3 and P4 genes for resistance. Segregation for virulence occurred on flax cultivars with the L, 11, L4 and L10 genes. Virulence on cultivars with the L4 and L10 genes was conditioned by single recessive gene pairs. The segregation ratio for virulence on the L1 gene more nearly fit a digenic recessive ratio than a monogenic ratio. Virulence on the cultivar containing the L gene appeared to be conditioned by 2 recessive gene pairs. Virulence on cultivars with gene L4 was apparently linked with L10, but no linkage was detected between L and L4 or L and L10.