Agrobacteriumtumefaciens (E. F. Smith and Town.) Conn, was evaluated as a promoter of wound healing on mechanically inflicted basal wounds on 50 greenhouse-grown and 45 field-grown yellow birch, Betulaalleghaniensis Britt. Inoculated and noninoculated wounds were wrapped with a clear plastic wrap. The healing process of external wound closure by callus and internal compartmentalization of tissues extant before wounding was observed. Treated wounds showed a fourfold to fivefold increase in wound closure after 30 days for greenhouse-grown trees and after 45 days for field-grown trees. Effects of internal defense reactions that occur after injury were histologically examined. Compartmentalization of the wounded tissue was stimulated by treatment with A. tumefaciens. Abundant vessel plugging and an enhanced zone of cells, which formed after wounding, served to compartmentalize the wound. Although the effect of A. tumefaciens against antagonistic microorganisms is uncertain, the natural defense reactions of the tree were intensified while accelerating wound closure.