A gene (xynA) encoding the endo-xylanase (E.C.3.2.1.8) from Bacillus stearothermophilus was cloned in E. coli, and its complete nucleotide sequence was determined. The xynA gene consists of a 636 base pairs open reading frame coding for a protein of 212 amino acids with a deduced molecular weight of 23, 283 Da. A putative signal sequence of 27 amino acid residues shows the features comparable with the Bacillus signal sequences; namely, the signal contains a positively charged region close to the N-terminus followed by a long hydrophobic string. The coding sequence is preceded by a possible ribosome binding site with a free energy value of -16.6 kcal/mol and the transcription initiation signals are located further upstream. The translation termination codon (TAA) at the 3 end of the coding sequence is followed by two palindrome sequences, one of which is thought to act as a terminator. The xynA gene has a high GC content, especially in the wobble position of codons (64%). Comparison of the primary protein sequence with those of other xylanases shows a high homology to the xylanases belonging to family G.