The crystal structure of modification III of 5-ethyl-5-phenylbarbituric acid has been determined and refined to a conventional R of 0.066 on 2296 reflexions. Form III is monoclinic, P2a/C; a= 9"534, b= 11"855, c= 10"794 A, t= 111"56°; Z= 4. Intensity data were collected on a four-circle diffractometer using Mo K~ radiation. The structure was solved by direct methods, the hydrogen atoms located, and the positions of all atoms except hydrogens were refined independently with anisotropic temperature factors. Molecules in the structure are linked together by pairs of hydrogen bonds into infinite chains lying along 2x axes. The conformation of the ethyl and phenyl groups is largely controlled by packing requirements.