The electron spin resonance of an X-ray irradiated single crystal of diketopiperazine has been measured at 9 Gc/sec for various orientations of the crystal in the static magnetic field. From an analysis of the hyperfine structure observed we have been able to calculate the components of the hyperfine interaction tensor for the radical produced by the irradiation. The principal values for the hyperfine interaction tensor are similar to those found for the radical produced by the X-ray or γ-ray irradiation of glycylglycine and N-acetylglycine and is therefore attributed to a radical with a C—H fragment. The fine details of the spectrum indicate that the radical responsible for the electron resonance observed probably has the structure [Formula: see text]which is formed from diketopiperazine by the loss of a hydrogen atom from one of the CH2 groups in that molecule.