Saccharomyces yeasts are currently used for the industrial production of ethanol by fermentation of glucose derived from corn grain.  These yeasts do not naturally consume the five-carbon sugars contained in the hemicellulose component of biomass feedstocks.  Enzymes from native pentose-assimilating yeasts have been transferred to S. cerevisiae allowing fermentation of xylose, the most abundant biomass-derived pentose sugar.  However, efficient conversion of xylose to ethanol is limited by cellular redox imbalance resulting from different co-factor specificities of the first two enzymes for xylose utilization (i.e. the xylose reductase and xylitol dehydrogenase enzymes).  Replacing these two enzymes with a single, co-factor independent, xylose isomerase (XI) is proposed to alleviate this redox imbalance and improve xylose fermentation.  Aside from an XI from the anaerobic fungus Piromyces, a very limited number of xylose isomerases have been demonstrated to function when expressed in S. cerevisiae.  The goal of this study was to identify additional novel XI’s that function in S. cerevisiae.  Xylose isomerase genes were isolated from multiple human and ruminant intestinal bacteria by PCR amplification with degenerate primers.  These XI genes were expressed from plasmids in a S. cerevisiae strain that also expressed the S. cerevisiae xylulokinase gene at elevated levels.  Xylose isomerase activities for these new enzymes and xylose fermentation data for the different XI-expressing S. cerevisiae strains will be presented.