The effects of aeration on the production of hemicellulases by Thermomyces lanuginosus SSBP in a bioreactor were investigated. Increased aeration rates induced higher beta-xylanase production, with the highest beta-xylanase activity of 6750 nkat/ml obtained at 0.75 vvm, within 48 h. This level was lower than the 13 500 nkat/ml obtained at 1 vvm. An increase in aeration rate up to 1.25 vvm in a separate bioreactor experiment resulted in a drop in beta-xylanase titre to 2500 nkat/ml. The highest volumetric productivity for beta-xylanase of 330000 nkat/l/h was obtained at both 0.5 vvm and 0.75 vvm. Soluble protein levels also increased with increasing aeration levels, with 0.45 mg/ml produced at 0,75 vvm and 0.6 mg/ml at 1.0 vvm, respectively. Maximum activities of beta-xylosidase (5.21 nkat/ml) and alpha-arabinosidase (0.79 nkat/ml) were obtained at 0.75 vvm, while beta-glucosidase (1.15 nkat/ml) and beta-mannosidase (0.40 nkat/ml) levels remained constant between 0.75 and 1.0 vvm. These results indicate that aeration rates greater than 1.0 vvm adversely affect beta-xylanase and accessory enzyme production. (C) 2002 Elsevier Science Ltd. All rights reserved.