Larvae and adults of the species Dermestes maculatus were attracted by palmitic and stearic acids and repelled by a lower homolog, caprylic acid. Lauric acid, which strongly deterred larvae, was phagostimulatory to adults. Palmitic and stearic acids enhanced larval growth and metamorphosis, whereas caprylic and lauric acids were inhibitory. The above effects parallel the feeding response. Larvae differed from adults in their response to alpha fluoro fatty acids. The former were invariably repelled by the fluoro analogs, whereas the latter were stimulated to feed on a diet containing alpha fluoro laurate and alpha fluoro stearate. It appears that mature stages were incapable of distinguishing between fatty acids and their respective fluoro derivatives. Sodium fluoro acetate was highly toxic to hidebeetle larvae, while alpha fluoro fatty acids appeared relatively nontoxic, inhibiting growth at a dietary level of 1.0% only. This moderate effect was ascribed to inhibition of beta oxidation, whereby the lethal fluoro acetate was not released.