Toxicity bioassays of the naturally occurring insecticide 2-tridecanone and a group of structural analogs against the tomato fruitworm (Heliothis zea) reveal a significant influence of chain length on toxicity of methyl ketones. This effect may be due to the differential ability of these compounds to penetrate lipid barriers and reach the active site, since a close relationship is seen between toxicity and lipophilicity. Congeners of chain lenths 15 and over are less active than predicted by this relationship, possibly due to steric effects.