Thermoanaerobacter ethanolicus is a Gram-positive thermophile that converts xylose to ethanol. A portion of the T. ethanolicus xylose transport permease gene (xylH) was cloned, and the deduced protein exhibited greater than 60% similarity to homologs in enterobacteria. Xylose-binding protein and (xylH) transcripts were quantitated and compared from cells grown in batch or continuous cultures grown on xylose, glucose, or a mixture of both sugars. In contrast to the strong repression of xyl operons by glucose in other bacteria, both xylF and xylH expression were detected in the presence of this hexose sugar. Expression of xylF and xylH generally increased with dilution rate (3 and 1.5-fold, respectively) and seemed to be growth rate rather that substrate dependent. Overall. these unusual sugar utilization patterns in batch and continuous culture seem to result from a basal expression level of genes in the absence of xylose. T. ethanolicus is unique in possessing a triumvirate of xylose transport and catabolism operons and, given its extensive hemicellulolytic capabilities. may have evolved to constitutively express xyl genes.