Previously no immunity or high degree of resistance to soybean rust has been reported. The tests described have demonstrated that strong resistance to rust does exist in soybean genotypes, and in particular in one accession of P1200492 (Q12956) and the cultivars derived from it (Tainung 3 and Tainung 4) and in lines P1227268, PI227687 and HY2217. However, these resistances may not be effective against the whole rust population, as evidenced by the identification of a second race of rust which is virulent on the previously immune line P1200492. Rust of soybeans, caused by Phakopsora pachyrhizi Syd., is reported to be widely distributed throughout Asia from China south to Australia (1, 6, 10). The rust causes premature defoliation of plants, with a subsequent increase in the numbers of unfilled pods per plant. Disease loss has not been accurately determined, but the data available show that this disease can cause serious yield loss. In Taiwan it is estimated that, due to rust, production is reduced by 20-30% (8), whilst in individual fields as much as 70-80% of potential yield may be lost (4). In Japan, losses of 15-40% in individual fields are reported (7), and in Thailand losses are estimated to range from 10-30% in individual fields (8). No accurate measures of yield loss have been reported for Australian crops. Screening tests of soybean accessions have been carried out in several countries where the rust is indigenous, to identify sources of resistance to rust. The tests have shown that accessions differ in the severity of rust development that occurs on them, and show varying amounts of field resistance. However, no soybean lines which are immune to the rust, or which show a high degree of resistance have been reported (1). Accession P1200492 (a) was shown to have a high degree of field resistance to rust when tested in Taiwan (2, 3), though it was infected and sporulation occurred readily in glasshouse tests made in the U.S.A. using a Taiwanese isolate (9). PI200492 was used subsequently as a parent to breed cultivars Tainung 3 and Tainung 4 which showed degrees of rust resistance in Taiwan (2, 3)