SUMMARYPure pathotype A populations ofHeterodera rostochiensisproduce a few females on exandigenahybrids with theH1gene for resistance. As the proportion of larvae able to become female on exandigenahybrids was not increased by reproducing the nematodes on such hybrids for 3 years, these females seem not to be genetically different from the rest of the population. The proportion increased rapidly when the initial population contained a few pathotype (species) E nematodes but again no increase in the proportion of pathotype (species) A larvae able to become female on exandigenawas detected and pathotype E replaced pathotype A.