The structures of the new pyrethroids were elucidated utilizing nuclear magnetic spectroscopy and elemental analysis. The biological activity of the pyrethroids has been evaluated for houseflies and yellow fever mosquitoes. A correlation of increase in rapid knockdown and toxicity with decrease in size of halogen has been made. The most effective compound, the difluorovinyl ester, has a toxicity of 123 times greater than pyrethrins. This degree of toxicity has not previously been reported for a 5-benzyl-3-furanmethyl ester. The relative lipophilicities of the dihalovinyl esters, Rm, have been determined by reverse phase partition chromatography. The R values for the dihalo esters correlate with their m effectiveness as rapid knockdown agents. The least lipophilic compound, the difluorovinyl ester, possessed the highest knockdown in this dihalovinyl ester series. The difluorovinyl ester has incorporated a high level of toxicity while maintaining excellent effectiveness as a knockdown agent against houseflies. This combination of high knockdown and high toxicity in a single compound has not been previously reported. Several other pyrethroids were synthesized and evaluated for knockdown and toxicity, in order to make specific comparisons with pyrethroids of known effectiveness. The replacement of the isobutenyl group of the chrysanthemates by the dihalovinyl group has been effective against houseflies and yellow fever mosquitoes in enhancing or maintaining toxicity and knockdown in every pyrethroid series investigated. In the allethronyl series, this dihalovinyl replacement has resulted in a 12-fold increase in rapid knockdown activity against mosquitoes.