The adenylate kinase was purified from an extreme thermophile by adenosine-pentaphospho-adenosine elution from phosphocellulose column. The molecular weight was estimated to be 22,000 by SDS-PAGE and gel filtration. The optimum temperature of the enzyme activity was 8$0^{circ}C$ and the activation energy was given as 22.4 kcal/mole. The enzyme even showed full activity after incubation at 9$0^{circ}C$ or in 6M guanidine-HCI at 3$0^{circ}C$ and retained 75% of its original activity even after 1 hour at 10$0^{circ}C$. The Michaelis constants of the enzymes for AMP, ADP, and ATP were 0.01mM, 0.017mM and 0.067mM, respectively.