The effect of Ga substitution for Cu in NdBa2Cu3-xGaxO7-δ (0≤x≤1.0) on the structure, electrical and magnetic properties and Tc behavior has been investigated. Solid solubility exists for 0≤x≤1.0 in contrast to the Ga-doped Y-123 system. The orthorhombic to tetragonal phase transition occurs at x=0.09 suggesting that Ga is substituting at the Cu(1) site. Estimation of oxygen content reveals that δ values lie in the range 0.05-0.15 for 0≤x≤1.0. Tc decreases systematically with increasing x and the rate of decrease of Tc (12 K/atom%) is much more than that for any trivalent ion substituted Y-123 systems hitherto reported in literature. The compounds with x≥0.3 (tetragonal structure) are semiconductors with low energy of activation. © 1992.