The Clostridium thermohydrosulfuricum xylA gene encoding xylose(glucose) isomerase was cloned in the yeast expression vector pMA91 under the control of the PGK promoter, resulting in pBX-1, and transformed into Saccharomyces cerevisiae. Production of recombinant xylose isomerase was seen in the a Coomassie stained SDS-PAGE gel and the molecular mass was estimated to be 43 kD. The recombinant xylose isomerase showed the highest activity at 85 degrees C and pH7. The specific activity under these condition was 1.0 U/mg protein. At 30 degrees C and 40 degrees C, the relative activity was reduced to 3.7% and 11%, respectively, of the maximum.