THE structure of unusual nucleic acid components is of importance in relation to conformational and model building studies on nucleic acids in general and those concerning tRNA in particular. In the case of 5-iodouridine (Fig. 1) it is particularly desirable to know the crystal structure in order to compare the effects of ionizing radiation on single crystals of 5-iodouridine with those on 5-iododeoxyuridine, using the technique of electron spin resonance1. Such studies may elucidate the effects of radiation on the nucleic acids themselves.