Thermophilic xylose isomerase from the xerophytic eukaryote Opuntia vulgaris can serve as a good alternate source of enzyme for use in the production of high fructose corn syrup. The existence of two temperature stable isoforms having optimal activity at temperatures 70 degrees C (T-70) and 90 degrees C (T-90), respectively, is reported here. These isoforms were purified to homogeneity using column chromatography and SDS-polyacrylamide gel electrophoretic techniques. Only the T-90 isoform was subjected to full biochemical characterization thereafter. The purified T-90 isoform was capable of converting glucose to fructose with high efficiency under the assay conditions. The enzyme at pH 7.5 exhibited a preference to yield the forward isomerization reaction. The melting temperature of the native enzyme was determined to be 90 degrees C employing differential scanning colorimetery. Thermostability of the enzyme protein was established through temperature-related denaturation kinetic studies. It is suggested that the thermostability and the wide pH activity of this eukaryotic enzyme will make it an advantageous and dependable alternate source of catalytic activity for protected use in the high fructose corn syrup sweetener industry.