This invention relates to new pyrimidine derivatives with herbicidal activity corresponding to formula I ##STR1## wherein X is halogen, lower alkoxy or lower alkylthio Y is hydrogen or halogen and ##STR2## are aliphatic amino radicals, one or both of which contain the cyclopropyl ring. A process for their manufacture is given. Herbicidal compositions which contain these new pyrimidine derivatives as active component and their use for the post-emergent total or selective combating of weeds are also disclosed.