Evidence has been obtained through crossing and selfing studies that some physiologic races of oat stem rust contain both homozygous and heterozygous lines, the latter when selfed tending to produce races more virulent than the selfed race. In crosses between races, the less virulent characteristics are generally dominant so that the hybrid race is often identical with the less virulent of the two parent races. Selfing studies and crosses therefore indicate that the more virulent characteristics of oat stem rust are recessive traits. Reciprocal crosses between physiologic races suggest that the cytoplasm, as well as the nucleus, contributes to the inheritance of certain pathogenic characters.In crosses between races of normal (red) and orange uredial colour, the red colour has invariably proved dominant, the progeny being indistinguishable in colour from the normal parent.Two selfing studies of Race 3 carried out with the same telial material at an interval of more than four years have provided evidence of a genetic change presumably brought about through the ageing of the teliospores. In the first selfing, no abnormalities were observed in the infections on the barberry. In the second selfing, about half of the infections developed pycnia and pycnial nectar, while the remainder appeared as small, round, necrotic areas showing no indication of even rudimentary pycnial formation.