The F2 generation of crosses between a number of varieties of wheat susceptible to the attacks of Yellow Rust (Puccinia glumarum) and “American Club.” which shows an extraordinary degree of resistance to the attacks of this parasite showed in the year 1906 obvious segregation into immune and susceptible forms. The immunity was further found to be independent of any recognizable morphological feature. An analysis of these F2 generations demonstrated the fact that the immune plants were present to the extent of 25 per cent., that is that the property of resisting the attacks of Yellow Rust behaves as a recessive character to the property of being susceptible. The plants which proved susceptible showed, however, very varied degrees of susceptibility. Many were as covered with rust pustules as Michigan Bronze, one of the extremely susceptible varieties used as a parent. Others on the other hand were relatively slightly attacked and between these extremes all degrees of susceptibility appeared to exist. Nilsson-Ehle who has also observed the segregation of this pair of characteristics has also noted the wide differences in susceptibility in plants of the F2 and succeeding generations.