Five races of Mayetiola destructor (Say) collected in 19 localities in 5 States had varying ability to survive on wheat cultivars: Vermillion (a soft wheat in which the source of resistance has not been identified), Monon with the H3 gene, and Knox 62 with the H6 gene. Race B, which is capable of infesting Vermillion and cultivars having the H3 gene, has replaced Race A as the predominant race in Indiana. This major change appears to be associated with the wide-spread use of resistant cultivars since 1955. Race D, a highly virulent race capable of infesting Vermillion and cultivars having either the H3 or the H6 genes, also was discovered in Indiana wheat fields for the first time. Race A was the predominant race in other eastern fields where soft wheat is grown and where the acreage of resistant cultivars is low. In these fields, Race B and Race C (capable of infesting Vermillion and cultivars having the H6 gene) were seldom found. In the hard-wheat region of Kansas, the Great Plains race that cannot live on any resistant cultivars predominated in 2 populations; few members of Race A occurred and Races B, C, and D were not found.