D-Xylulokinase (ATP: D-xylulose 5-phosphotransferase EC 2.7.1.17) was purified from a newly isolated thermophilic Saccharococcus caldoxylosilyticus in which the enzyme is constitutively expressed. The purified enzyme had a specific activity of 60 U/mg at 25degreesC, and 185 U/mg in its optimum temperature range of between 65 and 75degreesC. Its K-m for xylulose was 0.09 mM at 25degreesC. and for MgATP 0.16 mm, The molecular mass of the monomer was estimated to be 54 kDa, the holoenzyme comprising two subunits. Stability studies showed that the enzyme was stable up to 65degreesC, but denatured rapidly at 75degreesC in Tris buffer. However, it was stabilised by xylulose, which accounts for its high optimum temperature for activity. N-terminal amino acid analysis produced the sequence DHVIGVDLGTSAVKALLVD...which has 65% identity with two other Bacillus xylulokinases as deduced from their gene sequences, and somewhat less identity with other known xylulokinase sequences,