The dynamics of DNA molecules in agarose gel has been studied by measuring the birefringence in sine wave electric fields. It was found that the birefringence is the sum of a direct current (dc) component and several harmonics of the field frequency. A detailed study of these components was performed. It was found that the dynamics of DNA molecules under these conditions can be explained by a W conformation which is not detected under dc electric fields. The variation of the field frequency gives several relaxation modes in the birefringence signal, reflecting the known dynamic behavior of DNA molecules found in the reputation with elongation of the tube length regime.