The soft red winter wheat (Triticum aestivum L.) 'Caldwell' has effective resistance to leaf rust caused by Puccinia triticina. To determine the genetic basis of this resistance, Caldwell was crossed with the leaf rust-susceptible spring wheat 'Thatcher', and the F 1 plants were backcrossed to Thatcher to obtain backcross (BC) F 2 (BC 1F 2) families. In the seedlings, the BC 1F 2 families segregated for a single gene to P. triticina race SBDG, which is likely Lr14a based on tests with BC 1F 3 lines. In fi eld tests, the BC 1F 2 families segregated for a single gene for adult plant resistance that was independent of the seedling resistance. BC 1F 2 adult plants were inoculated in greenhouse tests with P. triticina races BBBD and THBJ, and resistant plants were selected and advanced by single-seed descent to produce BC 1F 4 lines. In greenhouse tests, adult plants of some of the BC 1F 4 lines had the same low and high infection type (IT) to different P. triticina races as the Thatcher isoline with Lr12. Other BC 1F 4 plants had intermediate IT to the P. triticina race THBJ. The BC 1F 4 plants with the intermediate ITs were selected and tested as BC 1F 5 adult plants in greenhouse tests and as BC 1F 4:6 lines in fi eld plot tests at two locations. Caldwell likely has the seedling resistance Lr14a, the adult plant gene Lr12, and an uncharacterized adult plant resistance gene that conditions an intermediate level of effective resistance in field plots.