The crystal structures of three compositions of Na+ beta "-aluminogallate with different gallium content (Na1+xMgx(Al1-yGay)(11-x)O-17, x = 0.67; y = 0.23, 0.30 and 0.41), have been studied by single-crystal X-ray diffraction (Mo K alpha) 298 K. The beta "-aluminogallates are isostructural with the beta "-aluminas. The gallium atoms occupy predominantly tetrahedral sites in the spinel blocks. These substitutions cause increases in the lengths of the a- and c-axes. The Na+ ion distribution in the conduction plane is influenced only slightly by increasing gallium substitution. The structural results are discussed in the light of earlier structural studies of Na+ beta "-alumina and Na+ beta "-gallate.