Abstract: A xylose isomerase gene from Thermus bacteria, such as Thermos aquaticus (ATCC 27634) and a gene having 60% or more of homology to the nucleotide sequence of Thermus aquaticus xylose isomerase gene of FIG. 1-3. A xylose isomerase from Thermus aquaticus characterized in that the xylose isomerase has the optimal pH of about 7, the stable pH range of from about 6 to 8.5, the optimal temperature of about 95.degree. C. and the molecular weight of about 44,000, and is stabilized with manganese or magnesium. A process for preparing a xylose isomerase comprising transforming a microorganism with a plasmid containing the above gene and a promoter, culturing the transformed microorganism and harvesting the produced xylose isomerase. A process for preparing fructose comprising isomerization of glucose to fructose in the presence of the above xylose isomerase.