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 $405\mathrm{nm}$ in a two-wave mixing experiment. The photorefractive gain coefficient was $0.47{\mathrm{cm}}^{-1}$ at $I=6.9\mathrm{W}/{\mathrm{cm}}^2$ , and the actual saturated value was probably larger than this. The time constant was $59\mathrm{ms}$ at $I=1.0\mathrm{W}/{\mathrm{cm}}^2$ . In addition to a refractive index grating, an absorption grating was also formed, which is attributed mainly to an ionized trap density modulation process.