The production of extracellular thermostable cellulase-free xylanase (EC 3.2.1.8) by Bacillus licheniformis A99 was studied in solid-state fermentation (SSF). Wheat bran was Sound to be the best substrate [16.3 U g(-1) dry bacterial bran (DBB)] among a few easily available lignocellulosics rested. The production of xylanase reached a peak in 72 h (16.8 U g(-1) DBB). A high level of enzyme (17.1 U g(-1) DBB) was produced in wheat bran moistened with tap water at a substrate-to-moistening agent ratio of 1:2.5 (w/v). The optimum temperature for xylanase production was 50 degrees C. An elevated water activity suited the enzyme production most. Maximum enzyme production was recorded ill an aerated glass column reactor (19.13 U g(-1) DBB). Repression by glucose was not evident in SSF even at 6% while it was seen in submerged fermentation (SmF) at or beyond 2%. Cultivation in large enamel trays yielded a xylanase titer (15-17 U g(-1) DBB in 72 h) that was comparable to that in flasks. The enzyme production was 22-fold higher in SSF than that in SmF. This is the first report on xylanase production by a thermophilic bacterial strain in SSF.