Wounding of explanted Pinus nigra primary explants followed by infection with Agrobacterium rhizogenes wild strains 8196, 15834, or with the pRiA4abc transconjugant strain of A. tumefaciens (C58 chromosomal background) resulted in adventitious root induction. Roots were formed in 60-97% of explants (1-3 roots/explant) but without a hairy root phenotype. The presence of T-DNA of pRi8196 or pRiA4abc in regenerated roots was confirmed by the opine (mannopinic acid) content. Transformation response was influenced by the bacterial strain, age of explant and period of co-cultivation. Both the aggregate state (liquid) of medium and the season of the year (spring) had a positive effect on the root induction and their development. Histological analysis of the transformed roots showed that complete elements of primary and secondary root structures were present but roots were always triarch or tetrarch in the central cylinder as opposed to the primary roots of the untransformed seedling wich are diarch.