The organization of the genes involved in xylose catabolism in a thermophilic Bacillus sp. LW2 was found to be different from that of mesophiles such as B. subtilis and Escherichia coli. We have reported previously the cloning and characterization of the xylose isomerase gene (xylA) from this thermophilic Bacillus sp. A xylose transport permease gene (xylP) was located immediately upstream of xylA. This gene encoded a hydrophobic protein (392 amino acids) with a calculated molecular weight of 42,307. Further upstream of xylP, another related gene xylO encoding a sugar transport ATP-binding protein was observed. A xylulose kinase gene (xylB) was identified downstream of xylA. Its open reading frame extended up to 1122 bp and represented a part (372 aa) of the enzyme. The order of the xyl genes in the thermophilic Bacillus sp. is therefore 5'-xylO (ATP-binding protein)-xylP (xylose permease)-xylA (xylose isomerase)-xylB (xylulose kinase)-3'. The polarity of expression of these four genes was found to be unidirectional from xylO to xylB.