Feeding behavior of fourth- and sixth-instar spruce budworm,Choristoneura fumiferanaClem., toward aqueous extracts of balsam fir (Abies balsamea(L.) Mill.) foliage from terminal and lateral shoots of young and old trees is described. Fourth instars prefer extracts from terminal shoots of young and old trees. This feeding preference is correlated with the content of N in the foliage and the feeding rate is correlated with the N/tannin ratio. Sixth instars show a strong preference for extracts from lateral shoots of young trees. Their feeding preference is correlated with the sucrose content of the foliage, whereas their feeding rate is correlated with the glucose content. Age-related variation in bud worm larval feeding preference may be an important evolutionary adaptation for the spruce budworm, enabling it to respond positively to naturally occurring seasonal changes in the nutritional quality of its host plant and to chemical changes induced by defoliation.