Hypocotyls of PI 227687 and ‘Davis’ soybean, Glycine max (L.) Merrill, were used in a dual-choice feeding preference assay with fifth-instar soybean loopers, Pseudoplusia includens (Walker). (Lepidoptera: Noctuidae). Constitutive resistance in hypocotyls of PI 227687 relative to ‘Davis’ soybean was consistent with the relative resistance previously found in fully developed plants. The chemical p -chloromercuriphenyl sulfonic acid elicited inducible insect resistance in both PI 227687 and ‘Davis’ hypocotyls. However, the induced insect resistance was greater in PI 227687 than in ‘Davis’ hypocotyls. Both constitutive and inducible resistances were associated positively with the glyceollin concentration in the assayed hypocotyls but not with the concentration of another isoflavonoid, coumestrol.