Explorations of the A(1+)-RE(3+)-Mo(6+)-O(2-) (A(1+) is an alkali metal cation, RE(3+) is a rare-earth metal cation) quaternary systems prepared by the high-temperature solution growth method led to the title structure, sodium erbium bis-(molyb-date), NaEr(MoO(4))(2). It is isostructural to the scheelite structure (CaWO(4)) and is composed of [MoO(4)](2-) tetra-hedra with symmetry and [(Na/Er)O(8)](14-) polyhedra. The [(Na/Er)O(8)](14-) polyhedron is a distorted tetra-gonal anti-prism, also with symmetry, with statistically mixed Na/Er atoms at its centre. There are two sets of Na/Er-O bond lengths [2.420 (4) and 2.435 (3) Å], but just one set of Mo-O bond lengths [1.774 (4) Å].