THE natural pyrethrins (see refs 1 and 2 for references) (for example, pyrethrin I (1)) and related synthetic insecticides3 (allethrin (2), bioallethrin (3), resmethrin (4), bioresmethrin (5), Cismethrin (6) and the ethanochrysanthemate (7)) are unstable in air and light (see refs 1 and 2 for references, and ref. 4). This property restricts their use, particularly against pests of agricultural crops, in spite of other favourable characteristics (see refs 1–3 for references) (outstanding potency against many insect species, rapid action, and low mammalian toxicity5). We describe here new synthetic esters, ten to one hundred times more stable in light than previous pyrethroids, yet as active against insects as bioresmethrin (5) and with low mammalian toxicity.