The purpose of this study was to determine whether defoliation, a condition of stress caused by Choristoneura fumiferana herbivory, induces a disturbed pathologic condition in presumably normal tissue produced by the affected balsam fir trees. Techniques used in this investigation included dry weight determination, light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and biochemical and physical analyses. Terminal branch sections that contained shoots with buds and needles were cut from the upper crown on opposite sides of the tree. Collections were made from several previously selected trees in Vermont during April, June and July. The collected materials were labelled, placed in plastic bags and returned to the laboratory for histological and electron microscopic studies. The shoots derived from buds of stressed trees were shorter and produced fewer needles than shoots derived from equivalent position in nondefoliated buds. Unlike July-collected buds, the April buds did not exhibit distinct apical meristem zonation. The ultrastructural study of defoliated April-collected buds differed from nondefoliated buds, in that nondefoliated buds, were very active prior to bud burst and shoot elongation. This activity was indicated by the substantial increase in the number of Golgi-associated vesicles, several large vacuoles with heavy electron dense granular inclusions, abundant rough endoplasmic reticulum, proplastids and cell plate formation. Defoliated bud cells, unlike the nondefoliated bud cells did not contain a high population of cells exhibiting Golgi-associated rich regions, mitochondria, and apparent ce 11 p 1 ate formation. The absence or reduced presence of these metabolic indicators, in defoliated Aprilcollected buds suggest that the defoliated buds are metabolically disturbed. The eventual elongation of shoots of reduced length and number of needles as observed in June-collected buds indicates that activity was not completely inhibited in defoliated buds. The smaller bud size in defoliated balsam fir could also mean that prior accumulation of substances by the bud was effectua1 in that bud development may have been delayed. These findings indicate that Choristoneura fumiferana herbivory alters bud development.