Abstract: In order to study the role of hormones in the morphogenesis of witches' broom, tissues near the base of axiltary buds from the healthy and diseased young trees of Paulewnia elongata S. Y. Hu and P. catalpifolia Gong Tong. Were collected and cultured in vitro in MS medium with benzyladenine (BA) and indo-3-acetic acid (IAA) at different levels. When the tissues of healthy plants were treated with relatively high concentration of BA (1.5—3.0mg/l) and low concentration of IAA (0.1mg/l) in MS medium, the proliferation of lateral shoots was promoted,while the rate of IAA become higher(1.5—2.0mg /l) and that of BA lower (0.1mg/l) for diseased plants, such growth was inhibited.Besides, the endogenous cytokinins and IAA in the healthy and diseased plants were extracted and isolated. Ethanolic extracts passed through ionexchange resin, and further purified and isolated by thin layer chromatography. Cytokinin was identified by means of Amaranthus bioassay and ultraviolet spectral analysis. The extracted cytokinins were basically indentical with that of standard zeatin and isopentenyladenine. The absorption peak was in the region from 270—280nm, and the biological activity of the extracts from the diseased plants was shown higher than that from the healthy.The preliminary results in bioassay by the wheat coleoptile straight-growth test showed that the activity of IAA extracted from diseased plant was weaker than that from the healthy plants. Electrophorsis analysis for the isoenzymes of peroxidase in the extracts from both healthy and diseased plants were compared.From these results, it is suggested that the morphogenesis of witches' broom is controlled by the interaction among the hormonal levels. Cytokinins play an important role in the promotion of the proliferation of axillary bud growth; but auxins inhibit it.