A novel method for the separation of polyacrylonitrile (PAN) fibrils from fibers by ultrasonic etching in a 90 wt % aqueous dimethylsulphoxide (DMSO) solution at 75 degrees C +/- 2 degrees C for 6 h with a frequency of 40 kHz is demonstrated. These fibrils with a diameter of about 450 nm were systematically investigated by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and wide-angle X-ray diffraction (WAXD). It was found that the fibrils consisted of microfibrils with about 200 nm diameter, including periodic lamellae with thickness of 30-45 nm perpendicular to the fiber axis. The PAN fiber crystallinity and crystal size slightly decreased under the ultrasonic etching. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 617-619, 2010