Laboratory studies showed that the soybean phytoalexin glyceollin, an isoflavonoid previously investigated as an inhibitor of fungal pathogens, is an effective antifeedant for some insect species. Glyceollins extracted from soybean,Glycine maxMerrill, cotyledons were applied to the surface of common bean leaves,Phaseolus vulgarisL., in five concentrations, including physiological concentration, for feeding preference tests. Leaves treated with glyceollin at concentrations below physiological levels were less acceptable to the southern corn root worm,Diabrotica undecimpunctata howardiBarber, and the Mexican bean beetle,Epilachna varivestisMulsant, than untreated leaves. Feeding deterrence was positively correlated with increasing concentrations. The bean leaf beetle,Cerotoma trifurcataForster, was not affected even by very high doses. An ethological concentration (EC50) was computed based on the log dose–reduction of acceptance of treated disks. The EC50for the Mexican bean beetle was 6.1 μg/mg leaf dry weight and 3.5 μg/mg for the southern corn rootworm. Results indicate that soybean phytoalexins may represent a common defense against microorganisms and insect herbivores.