We have purified and characterized the histone-like protein, termed HAN1, and an HAN1-associated DNA-binding protein (hDBP) from nucleoids of the hyper-thermophilic Thermococcus-like AN1. HAN1 is shown to be composed of two subunits, to be thermally stable and to compact DNA in a reversible manner. The N-terminal sequence of HAN1 shares a high degree of homology with HMf, the histone-like protein from Methanothermus fervidus. Consistent with this, the toroidal wrapping of DNA by HAN1 resembles that described for HMf. However, significant differences in both twist and writhe components of these complexes are indicated by the 12.0 bp helical repeat produced during hydroxyl radical footprinting with HAN1. Furthermore, the increased stability of HAN1: DNA complexes allows DNA to be protected from thermal denaturation and cleavage by the restriction enzyme TaqI at 65°C. The hDBP, which co-purified with HAN1, is shown to represent a major portion of the acid-washed nucleoid protein in AN1 and to enhance the mobility of DNA directly, yet decrease the mobility of HAN1:DNA complexes.