Molecular order in an amorphous polymer with anisotropic magnetic susceptibility is altered by applying external magnetic fields. Disks of atactic polystyrene (a-PS) are prepared by solvent casting outside or inside a magnet. The effect of the magnetic field on the polymer samples is investigated by magnetic levitation and solid state NMR spectroscopy. Magnetic levitation of the a-PS disks shows that the orientation of disk symmetry axis with respect to the magnetic field gradient depends on the magnitude and direction of the applied field during casting. Similarly, carbon-13 two-dimensional cross-polarization/magic angle spinning rotor-synchronized NMR measurements in these samples show modulation patterns of the spinning side bands only on disks prepared in the presence of a magnetic field. These findings suggest that macromolecular order could be induced in a fluid or fluid-solid phase transition with cooperative segmental motions reorienting the diamagnetic susceptibility tensor to minimize the magnetic contribution to free energy of the sample. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1009-1015, 2010