Crystals of alpha-S-cysteinylthymine, C8H12CIN3O4S, formula weight 281.72, are orthorhombic, space group P212121, with a=9.499 (1), b=24.072 (4), and c=5.012 (1) A, V=1146.1 (2) A3, and Z=4. The structure was determined by the direct method and refined by a full-matrix least-squares procedure to a final residual, R=0.043, using 1277 diffractometer data. From the structure, a three-dimensional model for the radiation-induced interaction of thymine residues and cysteine residues could be postulated.