Ultrasonically induced birefringence was observed on the entanglement networks of the rodlike micelles of hexadecyldimethylammine oxide and mixtures of hexadecyltrimethylammonium bromide and sodium salicylate. After the onset of ultrasound, the birefringence increased suddenly and approached a steady value after a damped oscillation. The steady value of the birefringence was proportional to the ultrasonic intensity and increased with the ultrasonic frequency. The birefringence of the entanglement networks is induced by the radiation pressure. The period of oscillation and damping time constant of birefringence were related respectively to the plateau modulus and the stress relaxation time obtained from rheological measurement.