The virulence ofPhytophthora infestans has been increased by passage through resistant varieties to a point where a number of hybrids which give an immune reaction in comparison with standard varieties were blighted severely. A similar build-up of virulence occurred under wholly natural conditions. A gap exists which has not been bridged either naturally or artificially so that many hybrids still exist which give no reaction whatever when inoculated with cultures of any virulence. Some speculation is indulged in regarding the case of Champion, the potato-tomato situation in North America, and the occurrence of biological specialization.