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 Ae. triuncialis. In situ hybridization using Ae. 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_2:3 population showed that the rust resistance was monogenically inherited. Homoeologous group-2 RFLP markers XksuF11, XksuH16, and Xbg123 showed diagnostically polymorphic alleles between the resistant and susceptible bulks. The alien transfer originated from homoeologous chromosome recombination. The Ae. triuncialis-specific alleles of XksuH16, XksuF11, Xbg123 and one SSR marker Xcfd50 co-segregated with the rust resistance suggesting that the wheat-Ae. triuncialis translocation occurred in the distal region of chromosome arm 2BL. This translocation was designated T2BS·2BL-2^tL(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