The intermetallic compounds FeGa 3 and RuGa 3 were prepared from the elements using a Ga flux and their structures were refined from single-crystal X-ray data. Both compounds crystallize with the FeGa 3 structure type (tetragonal, space group P4 2 /mnm, Z=4). Electrical resistivity measurements revealed a semiconducting behavior for FeGa 3 and RuGa 3 , which is in contrast to the good metallic conductivity observed for the isotypic compound CoGa 3 . The origin of the different electronic properties of these materials was investigated by firstprinciple calculations. It was found that in compounds adopting the FeGa 3 structure type the transition metal atoms and Ga atoms interact strongly. This opens a d-p hybridization bandgap with a size of about 0.31 eV in the density of states at the Fermi level for 17-electron compounds (i.e., FeGa 3 and RuGa 3 ). The electronic structure of CoGa 3 (an 18-electron compound) displays rigid band behavior with respect to FeGa 3 . As a consequence, the Fermi level in CoGa 3 becomes located above the d-p hybridization gap which explains its metallic conductivity.