Electrical conductivity sigma was studied in single- and polycrystalline lithium heptagermanate Li(2)Ge(7)O(15)doped with Al. In Li2Ge7O15:Al single crystal conductivity demonstrated strong anisotropy. As compared with pure crystal, doping significantly increased conductivity along the structural channels parallel toaandcaxes. The doping effect was interpreted by supposing that Al(3+)substituted for Ge(4+)hosts. Li interstitials (A(Li)) were considered as the defects compensating charge imbalance introduced by the impurity. Heterovalent substitution Al3+(Ge4+) noticeably increased concentration of A(Li)which were mobile enough and contributed to conductivity. Influence of Al doping on sigma was compared for Li(2)Ge(7)O(15)single crystal and polycrystal.