Effects of medium components on intracellular glucose isomerase (GI) production were investigated by Bacillus thermoantarcticus. The highest GI activity was obtained as 1630 U dm(-3) in the medium containing (g dm(-3)): 10.6, birchwood-xylan: 5.6, yeast extract: 5.9 (NH4)(2)SO4 at T = 55 C in 33 cm(-3) shake-flasks. When birchwood-xylan was replaced with oat spelt- or beechwood-xylan, GI activity decreased to 1372 and 1308 U dm(-3). respectively. Effects of pH at uncontrolled-pH (pH(UC) = 6.0) and controlled-pH (pH(C) = 6.0) operations, and oxygen transfer at the all, inlet rate of 0.5vvm and agitation rates of 300, 500 and 700 min(-1), were investigated in 3.0 dm(-3) bioreactor system with 1.65 dm(3) working Volume in the designed medium. The highest GI activity was attained at 500 min(-1), 0.5 vvm, pH(UC) = 6 as 1840 U dm(-3) where cell concentration was 2.3 g dm(-3). The use of agricultural waste xylan, as the carbon source resulted in concomitant production of xylanase and GI. The highest xylanase activity was attained as 9300 U dm(-3) at 500 min(-1) and 0.5 vvm. K(L)a varied between 0.008-0.033 s(-1) whereas the highest oxygen Uptake Fate was 0.002 mmol dm(-3) s(-1). Initially biochemical reaction limitations were effective: thereafter, mass transfer resistances became more effective, (C) 2008 Elsevier B.V. All rights reserved.