A variety of organophosphorus compounds, carbamates, chelators, sulfhydryl group reagents, ions, and potential competitive inhibitors were tested as in vitro inhibitors of trans-permethrin hydrolysis by midguts of the soybean looper, Pseudoplusia includens (Walker). The nonpolar (aryl or long chain alkyl) organophosphorus compounds were found to be the most effective inhibitors, with some having I 50s of less than 10 −10 M. Paraoxon analogs with O-alkyl groups that were ethyl or longer were approximately equally effective inhibitors, while -thiono analogs were much less effective than -oxon analogs. For both carbamates and chelators the most effective compounds were those possessing aryl moieties. For sulfhydryl group reagents, both ions (Hg +2, Cu +2) and aryl compounds (γ-bromonitroacetophenone, 4-chloromercuriphenylsulfonic acid) were effective. Except for Hg +2, and Cu +2, the ions tested were relatively inactive. Several substituted 3-phenylthio-1,1,1-trifluoro-2-propanones produced I 50s of less than 10 −7 M. The oxidative inhibitors, pyrethroids, and other esters, amides, and alcohols examined were only moderately active (I 50 > 10 −6 M) at best. Of the compounds tested topically, the most potent synergists were the arylchloroorganophosphates, although a chelator (dibenzoylmethane) and a sulfhydryl group reagent (γ-bromonitroacetophenone), and 3-phenylthio-1,1,1-trifluoro-2-propanone showed some activity. The synergists that were effective for trans-permethrin on P. includens, were generally effective for fenvalerate on P. includens and trans-permethrin on Heliothis virescens (F.) as well.