A pseudopotential has been introduced into NMR protein structure determination that effectively restrainsmolecular volume based on the observation that rigid, well-packed protein structures are approximatelyellipsoidal. Allowing an ellipsoidal shape is more general than the single approximately spherical shape imposedby the radius of gyration pseudopotential introduced previously. We demonstrate that this new gyrationvolume term improves structures both during the calculation of an initial unknown fold and during finalrefinement.