Two strains of flax rust,Melampsora lini, designated strains CH5and I, were found to possess a number of double-stranded (ds) RNAs. A third strain, LMS, lacked dsRNAs. Sexual transmission of the dsRNAs was investigated by examining the dsRNAs in progeny obtained by selling and by intercrossing strains CH5and I. The results allow the dsRNAs in each strain to be assigned to transmission units, with strain CH5possessing two such units (designated L and C) and strain I three units (L, A, and B). The L unit consists of a single large dsRNA (about 4800 base pairs), while the other units (A, B, and C) each consist of a set of five to seven smaller dsRNAs, which taken together, total approximately 8000–9000 base pairs. The L unit in both strains was transmitted to all sexual progeny, whereas the multisegment units, A, B, and C, were transmitted to only some of the progeny. The A and C units were transmitted only via recipient pycnia ("maternal" inheritance), but transmission of the B unit occurred through both donor and recipient pycnia. Asexual transmission did not always occur, since one of five sublines of strain I lacked the B set. The dsRNAs were shown to be "infectious," since strain LMS acquired dsRNAs when grown on the same plant as strain I.