Enoyl-acyl carrier protein reductase (ENR) catalyzes the NADH-dependent stereospecific reduction of alpha,beta-unsaturated fatty acids bound to the acyl-carrier protein. ENR from Helicobacter pylori has been overexpressed in Escherichia coli and has been crystallized in the presence of its cofactor NADH and the inhibitor triclosan (or its analogue diclosan) at 296 K using polyethylene glycol (PEG) 400 as a precipitant. For the triclosan (or diclosan) complex, diffraction data to 2.5 (or 2.3) A resolution have been collected using synchrotron X-rays. The crystals belong to the monoclinic space group P2(1), with unit-cell parameters a = 73.35, b = 94.91, c = 75.38 A, beta = 106.21 degrees for the triclosan complex (or a = 73.25, b = 95.07, c = 75.02 A, beta = 106.53 degrees for the diclosan complex). The asymmetric unit contains one homotetramer, with a corresponding V(M) of 2.10 A(3) Da(-1) and a solvent content of 41% by volume.