AC 217,300 was highly toxic by topical application to adult Musca domestica and Blattella germanica, and larvae of Spodoptera eridania, with LD₅₀ values in μg/g of 20, 19, and 61, respectively. It was relatively nontoxic to Diabrotica undecimpunctata howardi beetles and Heliothis virescens larvae with LD₅₀ values of 165 and >883. When incorporated into baits, AC 217,300 was still highly toxic to M. domestica, B. germanica, and S. eridania. A significant increase in toxicity to H. virescens was observed although the percentage mortality was only 45%. AC 217,300 was not effective against D. undecimpunctata with either mode of application. A comparison of toxicity with tissue residues indicated that AC 217,300 was highly toxic to M. domestica, B. germanica, H. virescens, and S. eridania if it was absorbed. However, D. undecimpunctata appeared to be much less sensitive to AC 217,300, as only 18% mortality was observed with corresponding tissue concentrations two to six times those producing 95–100% mortality in M. domestica and S. eridania. Exposure of M. domestica to an AC 217,300-treated bait for 3–6 hr was sufficient to cause 50–100% mortality after 2 days and 95–100% mortality by 7 days. With B. germanica, a 1- to 3-day exposure resulted in 75–90% control within 7 days, while exposure for 3–7 days gave 90–100% control. Environmental temperature had a pronounced effect on the toxicity of AC 217,300 to both M. domestica and B. germanica. A 20-fold increase in toxicity was observed when the environmental temperature was raised from 21 to 32°C. This observation, in concert with the symptoms of intoxication, support the hypothesis that AC 217,300 is an inhibitor of energy production. Approximately 85 and 87% of the total residue of AC 217,300 extracted from treated M. domestica and B. germanica, respectively, was parent material. Six minor metabolites were detectable, of which three were tentatively identified by cochromatography with authentic standards. None of the identified metabolites had any insecticidal activity in vivo, suggesting that AC 217,300 is the actual toxicant.