AbstractThe potency of six dietary pyrethroids, as toxicants and inhibitors of weight gain in first‐ and fourth‐instarTribolium castaneum(Herbst) larvae, decreased in the order ofcis‐cypermethrin and deltamethrin>trans‐cypermethrin andcis‐permethrin>fenvalerate andtrans‐permethrin. Dosages that reduced larval weight also delayed pupation and emergence, probably due to their antifeeding activity. Three oxidase inhibitors (piperonyl butoxide,O, O‐diethylO‐phenyl phosphorothioate, andO‐isobutylO‐prop‐2‐ynyl phenylphosphonate), at a dietary concentration of 100 mg kg−1, had little or no effect on the toxicity oftrans‐permethrin, but strongly synergised the toxicity ofcis‐cypermethrin by about 3‐, 3‐ and 10‐fold, respectively. Piperonyl butoxide also synergised the toxicity ofcis‐permethrin,trans‐cypermethrin and deltamethrin, but not that of fenvalerate. On the other hand, an esterase inhibitor, profenofos, did not enhance the potency of any of the α‐cyano‐3‐phenoxybenzyl pyrethroids. Oxidases appear to be more important than esterases in