The tetrameric, NADH-dependent enoyl acyl carrier protein reductase from developing seeds of Brassica napus (oil seed rape) has been crystallized from solutions containing ammonium sulphate as the precipitant in the presence of NAD+ or NADH using the hanging drop method of vapour diffusion. The crystals belong to the tetragonal system and are in space group P4(2)2(1)2 with cell dimensions a = b = 70.5 A, c = 117.8 A. Considerations of the possible values of Vm indicate that the asymmetric unit contains a single subunit. The crystals are resistant to radiation damage and X-ray diffraction photographs taken with synchrotron radiation show measurable reflections to beyond 1.9 A resolution. Determination of the structure of this enzyme will advance the understanding of the mechanisms of lipid biosynthesis in plants and provide an opportunity to study the interactions between this enzyme and its acyl carrier protein substrate.