The widely cultivated hybrid poplar Populus alba × P. glandulosa in Korea was transformed with Agrobacterium rhizogenes agropine type strain A4. Genetic transformation was confirmed by the presence of agropine. Alteration in growth rate of hairy roots was seen following changes in the dilution rate of medium and concentration of sucrose, suggesting that improved growth might be achieved by more precise manipulation of the nutrient medium. Plant regeneration occurred from transformed hairy roots on MS medium supplemented with 0.5 ㎎/ℓ BAP. Transformed plantlets grown in vitro exhibited a more developed root system characterized by fast growth behavior in comparison to normal plantlets. This work demonstrates that the root transformation would be useful in improving plantlet establishment and growth through the effective root system.