Phase gratings have been recorded in ferroelectric triglycine sulphate crystals doped with iron by interference of two ultraviolet laser beams in the 350 nm wavelength range. The recording sensitivity increases with temperature and a diffraction efficiency exceeding 60% has been reached in a 5.5‐mm‐thick crystal after several hours of writing with intensities of 0.1 W/cm2. The photoinduced refractive index changes are anisotropic. The recording mechanism is not connected with the photorefractive effect and is believed to be due to the redistribution of hydrogen bonds leading to structural and refractive index modifications.