The sharp limitation of synergistic action when equimolecular proportions of the pyrethrins and some of their synergists are tested against flying mosquitoes has been confirmed when such mixtures are employed in assays usingCalandra granariaL. crawling on oil‐films on filter‐paper.Limitation of the relative potencies of pyrethrum‐synergist mixtures appears to be a general property, and except for piperonyl butoxide the limiting relative potency is attained at equimolecular proportions of insecticide and synergist.These observations are discussed in connexion with a general theory of insecticidal a