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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 BC3F11 from a cross of hexaploid wheat and A. triuncialis. In situ hybridization using A. triuncialis genomic DNA as a probe failed to detect the alien intro- gression. The translocation line was resistant to the most prevalent races of leaf rust in India and Kansas. Genetic mapping in a segregating F2:3 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·2BL-21L(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. [ABSTRACT FROM AUTHOR]