We report our observation of the photorefractive effect in undoped aluminum nitride. We measured the coupling constant and the formation rate as a function of pump intensity at a wavelength of 405 nm in a two-wave mixing experiment. The photorefractive gain coefficient was 0.47 cm(-1) at I = 6.9 W/cm(2), and the actual saturated value was probably larger than this. The time constant was 59 ms at I = 1.0 W/cm(2). In addition to a refractive index grating, an absorption grating was also formed, which is attributed mainly to an ionized trap density modulation process.