The hypothesis that root suckering in the aspens is controlled by apical dominance was tested. Root pruning and girdling stimulated sucker formation on portions of root systems distal to the point of treatment. Suckering of root cuttings was reduced by treatment with aqueous solutions of IAA. IAA-lanolin paste inhibited sucker formation on intact root systems of 1-year-old aspen. It was concluded that the physiological mechanism controlling root suckering is related to the apical dominance phenomenon.