Rubredoxin (D.g. Rd), a small non-heme iron-sulfur protein shown to function as a redox coupling protein from the sulfate reducing bacteria Desulfovibrio gigas, has been crystallized using the hanging-drop vapor diffusion method and macroseeding method. Rubredoxin crystals diffract to an ultra-high resolution 0.68 Angstrom using synchrotron radiation X-ray, and belong to the space group P2(1) with unit-cell parameters a = 19.44Angstrom, b = 41.24Angstrom, c = 24.10Angstrom, and beta = 108.46degrees. The data set of single-wavelength anomalous dispersion signal of iron in the native crystal was also collected for ab initio structure re-determination. Preliminary analysis indicates that there is one monomer with a [Fe-4S] cluster in each asymmetric unit. The crystal structure at this ultra-high resolution will reveal the details of its biological function. The crystal character and data collection strategy for ultra-high resolution will also be discussed. (C) 2003 Elsevier Inc. All rights reserved.