By means of neutron diffraction the low-temperature crystal structure of NaBD4 has been determined. At 10 K the lattice parameters are a = 4.332(1) Angstrom and c = 5.869(1) Angstrom. Deuterium is found in a tetrahedral arrangement [sites (8g)] around B. The symmetry corresponds to space group P4(2)/nmc. For room temperature the structure model for NaBD4 of Davis and Kennard with disordered deuterium distributed over two sites has been revised to space group Fm (3) over barm. Thus the 190 K phase transition known from specific heat measurements is of order-disorder type, caused by reorientations of BD4 tetrahedra.