Mixtures of races of oat stem rust (Puccinia graminis Pers. f. sp. avenae Eriks. and E. Henn.), differing in virulence, were cultured on susceptible plants in growth cabinets for five uredial generations at 15, 20, and 25 °C and in the field. The races with fewest genes for virulence maintained or increased their levels in the growth cabinets in all cases but were consistently outperformed by races with more genes for virulence under field conditions. The field results are in general agreement with the course of evolution of physiologic races of oat stem rust in Canada over 50 years; they suggest that the number of genes for virulence carried by a given race, other than those required for successful parasitism, is probably not the key determinant affecting its success in nature.