A genetical study of resistance of wheat to black stem rust, and a plant breeding attack on the rust problem are described. A large F2population of the cross Vernal (T.dicoccum) × Marquis (T.vulgare) was grown under severe natural epidemic conditions in the field and hundreds of F3progenies were exposed in the seedling stage, under controlled conditions, to pure physiologic forms of rust. In the field Vernal is highly resistant and Marquis susceptible to most forms of stem rust. Resistance in the field proved incompletely dominant and appeared to be governed by a single genetic factor. Marquis and Vernal were found to differ by one main genetic factor, Rb, for seedling reaction to form 21. This factor Rb, carried by Vernal, also governs seedling resistance to forms 17, 29 and 36 and appears to be responsible for the slight seedling resistance of Vernal to form 27. There was some evidence that the factor Rbis the same factor that controls the resistance of the F2plants to the forms of rust in the field (forms 17, 21, 29 and 36 were known to be present.) A different factor Racauses the resistance of Marquis seedlings to form 27. Vernal resistance was not found to be associated closely with the seed shape of that variety nor with its adherence of glumes to the seed.