The malonyl coenzyme A-acyl carrier protein transacylase, a single polypeptide chain of 358 amino acid residues and a molecular mass of 32 kDa, is a key component of the fatty acid synthase multienzyme complex. The elucidation of its three-dimensional structure will help in the understanding of the molecular basis of the biosynthesis of fatty acids, as well as of polyketides and related biologically active molecules. Three X-ray-quality crystal forms of the Escherichia coli fabD gene product encoding for malonyl coenzyme A-acyl carrier protein transacylase have been obtained using the hanging-drop method and ammonium sulfate as precipitant. Two are tetragonal and each contains two molecules in the asymmetric unit (form I: space group P4(3(1))2(1)2 with a = b = 83.9 A, c = 166.5 A and form II: space group P4 with a = b = 132.64 A, c = 38.85 A), whereas the third form belongs to the hexagonal system and contains one molecule in the asymmetric unit (space group P6(1(5)) with a = b = 68.52 A, c = 117.71 A). In each case, the diffraction pattern extends to approximately 2.0 A resolution using CuK alpha radiation from a rotating anode source.