The electron spin resonance of an x-ray irradiated single crystal of α-glycylglycine has been measured at 9 gc/sec for various orientations of the crystal in the static magnetic field. The resonance pattern was found to be a doublet for all orientations and from the measurement of the anisotropy of the doublet splitting it has been possible to calculate the coupling tensor for the σ-hydrogen attached to a carbon atom in the radical. Further it has been established that the main radical produced by x-ray irradiation is H3N+CH2CONH ĊH CO2-. It has also been shown that the σ-CH bond in the radical must lie within 2° of the b axis of the monoclinic host crystal and that the ‘molecular’ plane of the radical is inclined at an angle of about 7° to the b-c plane of the host crystal.