Technical Abstract: Two soybean accessions, PI 587886 and PI 587880A, previously identified as having resistance to Phakospora pachyrhizi Syd. (soybean rust, SBR) were used to create two populations (POP-1 and POP-2) segregating for SBR resistance. F2-derived F3 families from each population were grown in a naturally SBR-infected field in Paraguay to determine inheritance and map resistance genes. Over 6,000 plants from 178 families in POP-1 and over 5,000 plants from 160 families in POP-2 were evaluated at R5 for lesion type: immune reaction (IR), reddish-brown (RB), or tan (TAN) colored lesions. Based upon the lesion type present, each F2:3 family was rated as immune, segregating or susceptible and this classification was used to infer the F2-genotype. For both populations the segregation ratios fit a 1:2:1 (immune:heterozygous:susceptible) ratio expected for a single gene (P>0.05). The RB lesions occurred almost exclusively in the heterozygous class indicating incomplete dominance under the conditions of this study. Molecular markers were used to map the location of the resistance gene to the Rpp1 locus in both populations. However, evaluation of PI 587886 and PI 587880A against eight P. pachyrhizi isolates indicated that the resistance allele in these two accessions was different from Rpp1. This test also demonstrated that these accessions were resistant to at least one P. pachyrhizi isolate collected in the southern US. This is the first report of using an adult plant field-screen with natural rust pressure to determine SBR inheritance and map SBR resistance.