A histone-like protein (HTa) has been isolated from cell extracts of Thermoplasma acidophilum by column chromatography on DNA-cellulose, hydroxylapatite, and Sephadex G-75, HTa elutes from DNA-cellulose in two fractions, one of which contains an 80-residue form of the protein with an NH2-terminal sequence of Val-Gly. The other fraction apparently contains the 89-residue species, in addition to a 90-residue form of the protein with the NH2-terminal sequence Met-Val. The sequence of 47 residues from the NH2 terminus of the 89-residue protein was established by automated Edman degradation. HTa is characterized by the following properties: 22% of its residues are lysine and arginine; the lysine:arginine ratio is 2.33; the absorption spectrum of the protein is distinctive due to the lack of tryptophan and the presence of 1 tyrosine and 5 phenylalanine residues; and the protein stabilizes DNA against thermal denaturation (Stein, D. B., and Searcy, D. G. (1978) Science 202, 219-221) and condenses DNA into spherical particles. All of these characteristics indicate that HTa resembles eukaryotic histones, but there are distinctive differences.