Starting from the known average structure (Dubbeldam & de Wolff, Acta Cryst. (1969), B25, 2665-2667) and four-dimensional space group, the actual modulated structure has been determined by taking into account 758 newly measured main reflexions and 1152 first-order satellites. The analysis is based on a harmonic displacive modulation model, with 14 modulation parameters as well as 9 positional and 26 thermal parameters (individual anisotropic, in Debye-Waller factors of the usual type).The final value of YA/Y~Fo is 0.108 excluding, and 0.238 including non-observed reflexions. The main features of the modulation are (a) an overall modulation with an amplitude of close to 0.33 A, for all atoms except oxygen; (b) an orientational modulation of the COa ion which is out of phase with the overall modulation. Calculation of the interatomic distances yields the result that the Dab symmetry of the CO3 ions is hardly distorted by the modulation. Certain second-order effects point to anharmonic components in the modulation, with amplitudes up to 0"03 A,. The structure turns out to consist of chains of NaO6 octahedra sharing faces and with Na-O bond lengths under 2"45 /~, showing little variation. The chains run in the direction of c and are mutually coupled in other directions by CO3 ions. The remaining Na atoms occupy the remaining spacious voids; their distances (averages > 2.60/~) to eight out of the nine surrounding oxygen atoms vary strongly, and for each with a different phase.