The properties of transgenic aspen (Populus tremula) clones carrying the recombinant gene of xyloglucanase sp-Xeg from Penicillium canescens have been analyzed. Complex modifications were revealed both in the composition of the wood and in the plant phenotype. Biometric analysis showed that shoot dimensions increased by 24.8%, 25% and 26% in the PtXIV-Xeg1a, PtXVXeg1a and PtXVXeg1b lines, respectively. The number of internodes in some transgenic clones also increased. Modifications in rhizogenesis have been shown for the first time in the plants with the recombinant gene of xyloglucanase: in vitro rooting efficiency exceeded the control value in 13 out of 25 lines. Maximum rooting efficiency was observed in the PtXVXeg1a line (3.2-fold higher than in the control). A reliable increase in the root system mass (by 20% to 52%) under greenhouse conditions was observed for 8 out of 25 clones. A lower pentosan content in the wood was shown for all lines. The data on xyloglucanase activity and pentosan content generally correlated with phenotypic modifications.