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Genes transferred to crop plants from wild species are often associated with deleterious traits. Using molecular markers, we detected a cryptic introgression with a leaf rust resistance gene transferred from Aegilops triuncialis L. into common wheat (Triticum aestivum L.). One agronomically desirable rust-resistant introgression line was selected and advanced to [BC.sub.3][F.sub.11] from a cross of hexaploid wheat and A. triuncialis, n situ hybridization using A. triuncialis genomic DNA as a probe failed to detect the alien introgression. The translocation line was resistant to the most prevalent races of leaf rust in India and Kansas. Genetic mapping in a segregating [F.sub.2:3] population showed that the rust resistance was monogenically inherited. Homeologous group 2 restriction fragment length polymorphism markers XksuF11, XksuH16, and Xbg123 showed diagnostically polymorphic alleles between the resistant and susceptible bulks. The alien transfer originated from homeologous chromosome recombination. The A. triuncialis-specific alleles of XksuH16, XksuF11, Xbg123, and one simple sequence repeat marker Xcfd50 cosegregated with the rust resistance, suggesting that the wheat-A, triuncialis translocation occurred in the distal region of chromosome arm 2BL. This translocation was designated T2BS.2BL2tL(0.95). The unique source and map location of the introgression on chromosome 2B indicated that the leaf rust resistance gene is new and was designated Lr58.