Deoxycytidylate hydroxymethylase from bacteriophage T4 is a homodimeric enzyme in which each polypeptide chain consists of 246 amino-acid residues. It has been crystallized in the presence of its substrate, deoxycytidine monophosphate, at room temperature using sodium citrate as precipitant. The crystals are monoclinic, belonging to space group C2, with unit-cell parameters a = 174.22, b = 53.12, c = 75.17 A, beta = 115.29 degrees. The asymmetric unit contains one homodimer, with a corresponding Vm of 2.65 A3 Da-1 and solvent content of 54%. Native diffraction data to 1.6 A resolution have been collected from two crystals using synchrotron radiation.