Arthrobacter strain N.R.R.L. B3728 superproduces a D-xylose isomerase that is also a useful industrial D-glucose isomerase. The gene (xylA) that encodes it has been cloned by complementing a xylA mutant of the ancestral strain, with the use of a shuttle vector. The 5' region shows strong sequence similarity to Escherichia coli consensus promoters and ribosome-binding sequences and allows high levels of expression in E. coli. The coding sequence shows similarity to those for other D-xylose isomerases and is followed by 22 nucleotide residues with stop codons in each reading frame, a good 'consensus' ribosome-binding site and an open reading frame showing similarity to those of known D-xylulokinases (xylB). Studies on the expression of the cloned gene in Arthrobacter and in E. coli suggest that the two genes are part of a xyl operon regulated by a repressor that is defective in strain B3728. Codon usage in these two genes, and in another open reading frame (nxi) that was adventitiously isolated during early cloning attempts, shows some characteristic omissions and a strong G + C preference in redundant positions.