Sitophilus zeamais Motschulsky and Tribolium castaneum (Herbst) adults were equally susceptible to contact toxicity of either safrole or isosafrole. Safrole also possessed equal fumigant toxicity to adults of both species. However, adults of S. zeamais were twice as tolerant as T. castaneum to the fumigant toxicity of isosafrole. In both contact and fumigant tests, 12- and 14-d-old larvae of T. castaneum showed similar susceptibility to each of the compounds, but 16-d-old larvae became more tolerant than the younger larvae. Isosafrole was generally more toxic than safrole to the insects tested. In flour disk bioassays, safrole showed very slight feeding deterrence against S. zeamais adults, and it had no feeding deterrence against T. castaneum adults and larvae. In contrast, isosafrole was a more effective feeding deterrent than safrole, with feeding deterrence indices of 31% for S. zeamais adults at 5.4 mg/g of food, and 72 and 40% for T. castaneum adults and larvae, respectively, at 10.8 mg/g of food. It reduced relative growth rate, relative consumption rate, and efficiency of conversion of ingested food of adults of both species, but only relative growth rate and relative consumption rate of T. castaneum larvae. Isosafrole was a stronger feeding deterrent against S. zeamais than against T. castaneum. Both of the compounds showed inhibitory effects on -amylase activity from T. castaneum larvae in vitro. Safrole also inhibited the larval -amylase in vivo. However, isosafrole inhibited -amylase activity at the lower concentration but activated it at the higher concentration in vivo.