Recent work has shown that the NADH-dependent enoyl acyl carrier protein reductase from Escherichia coli is the target for diazaborine, an antibacterial agent. This enzyme has been crystallized by the hanging-drop method of vapour diffusion complexed with NAD(+) and in the presence and absence of a thieno diazaborine. The crystals grown in the absence of diazaborine (form A) are in the space group P2(1) with unit-cell dimensions a = 74.0, b = 81.2, c = 79.0 A and beta = 92.9 degrees, and with a tetramer in the asymmetric unit, whilst those grown in the presence of diazaborine (form B) are in the space group P6(1)22 (or P6(5)22) with unit-cell dimensions a = b = 80.9 and c = 328.3 A, and with a dimer in the asymmetric unit. The structure determination of this enzyme in the presence of diazaborine will provide information on the nature of the drug binding site and contribute to a programme of rational drug design.