Single crystals suitable for X-ray diffraction studies have been obtained from a soluble Escherichia coli NAD(P)H-dependent quinone oxidoreductase (QOR) of molecular mass 35 kDa. Crystals were grown by the vapour diffusion method in the presence of NADPH and diffract strongly to beyond 2·0 Å. The space group was determined to be P212121 with unit cell dimensions of a=107·44 Å, b=104·06 Å, c=77·45 Å. There is one dimer in the asymmetric unit. © 1994 Academic Press Limited.