Synergism between nicotine and pyrethrum applied by injection to adultOncopeltus fasciatusDal. has been reported by Turner (1951).When these insecticides were applied alone and as a mixture to adultTribolium castaneumHbst., using a dipping technique, the data indicated that independent joint action occurred. Similar action could be eliminated because the two insecticides had varying relative potency.Since the effect of the pyrethrins has been short‐lived in some insects, it was postulated that the absence of synergism might be caused by failure of the nicotine to reach a site of action while the pyrethrins were still acting.Application of nicotine, followed later by treatment with pyrethrins, gave evidence of synergism. A test in which the interval between treatments was varied from ¾ to 6 hr. showed that toxicity was greatest with the shortest interval between applications, and evidence of synergism had practically disappeared after 6 hr. The maximum amount of synergism observed was about twofo