Carbamoyl Phosphate synthetase catalyzes the formation of carbamoyl phosphate, a high-energy intermediate used in several biosynthetic pathways. The enzyme from Escherichia coli has been crystallized at pH 8 in the presence of L-ornithine, MnCl(2) and ADP, using PEG 8000 in combination with NEt(4)Cl and KCl. The crystals (apparently) belong to the orthorhombic space group P2(1)2(1)2(1) with unit-cell dimensions of a = 154.4, b = 166.5 and c = 338.7 A. The crystals are relatively sensitive to radiation damage, but show diffraction to beyond 2.8 A resolution. A low-resolution (3.5 A) native data set has been recorded and conditions for flash cooling the crystal have been established.