Formulations containing various combinations of R-ll (2,3,4,5-bis(∆2-butylene)-tetrahydrofurfural), R-326 (di- n -propyl isocinchomeronate), MGK 264® ( n -(2-ethylhexyl)-5-norborneue-2,3-dicarhoximide), Tabutrex® (dibutyl suceinate), butoxy polypropylene glycol, and piperonyl butoxide with 0.02% pyrethrins and without pyrethrins were much less effective against stable flies ( Stomoxys calcitrans (L.)) and horn flies ( Haematobia irritans (L.)) than 0.1% pyrethrins without additives. When sprayed on a single animal, effectiveness of treatments appeared to be more greatly influenced by the quantity of pyrethrins in the formulation than by the presence or absence of additives. In topical-application tests the toxicity of pyrethrins to stable flies was slightly enhanced by the addition of either 10 X or 3.33 X piperonyl butoxide but not by the addition of MGK 264. Evidently, pyrethrins are more difficult to synergize against stable flies and horn flies than against house flies ( Musea domestica L.).