CyaY is a 106-residue protein from Escherichia coli. It shows amino-acid sequence similarity to human frataxin and a frataxin homologue in Saccharomyces cerevisiae, Yfh1p. The former is associated with the disease Friedreich ataxia and the latter plays a key role in iron homeostasis in mitochondria. CyaY has been overexpressed in soluble form in E. coli. The recombinant protein with a His(6) tag at its C-terminus has been crystallized at 296 K using polyethylene glycol (PEG) 4000 as a precipitant. Native diffraction data have been collected to 1.8 A using Cu Kalpha X-rays. The crystals belong to the trigonal space group P3(1)21 (or P3(2)21), with unit-cell parameters a = b = 44.66, c = 99.87 A, alpha = beta = 90.0, gamma = 120.0 degrees. The asymmetric unit contains one molecule of recombinant CyaY, with a corresponding V(m) of 2.13 A(3) Da(-1) and solvent content of 42.3%.