Two clones of Populus nigra L. were tested in vivo and in vitro for their susceptibility to three different Agrobacterium tumefaciens wild-type strains evaluating number and size of resulting calluses. Strain C58 proved to be the most virulent. Various parameters affecting Agrobacterium-mediated transformation of P. nigra clones were further analysed using b-glucuronidase gene transient expression. The clone Jean Pourtet proved to be more susceptible than the clone San Giorgio. A. tumefaciens strain A281 pKIWI105 proved to be the most virulent. The optimal procedure involved dipping of leaf discs into a bacterial suspension (7x108 cells/ml) for 20 min, followed by a 48 h co-cultivation period on semi-solid regeneration medium. Leaf explants were co-cultivated with two disarmed A. tumefaciens strains. Plantlets of San Giorgio were regenerated, tested for b-glucuronidase activity and rooted on selective medium containing kanamycin. Polymerase chain reaction analysis and Southern blot hybridization confirmed the integration of the neomycin phosphotransferase II gene into the poplar genome