The DNA-binding protein HU is ubiquitous in the prokaryotic cell. It is a major protein component of isolated nucleoids and is believed to control the tertiary structure of prokaryotic DNA. The Bacillus stearothermophilus HU (BstHU) mutants obtained by mutagenesis have been investigated. Crystallization experiments of BstHU-K38N (Lys38 is substituted with Asn) resulted in two forms of crystals suitable for high-resolution x-ray analysis. The first form belongs to the monoclinic space group C2 with unit-cell dimensions of a = 90.1 Angstrom, b = 43.5 Angstrom, c = 63.7 Angstrom, and beta = 135.1 degrees, and it diffracts x rays to 1.5-Angstrom resolution. The second form belongs to the tetragonal space group I4(1) with a = b = 62.6 Angstrom and c = 43.3 Angstrom, and it diffracts up to 1.8-Angstrom resolution,