The Presidential Address given by C. P. Clausen to the 62nd Annual Meeting of the Association of Economic Entomologists and recently published in the Journal of Economic Entomology (1) contains much food for thought for workers in the field of biological control. The thesis developed by Clausen in his address is, briefly, that any introduced parasite or predator that is destined to produce full commercial corkrol wiIl do so within three generations or at most within three years. If it does not produce full commercial control within this period it is safe to say that it will never do so. Species that prove ineffective after three generations or three years from the period of introduction should therefore be dropped and efforts directed toward the introduction of other species.