The effect of various carbohydrates on the production of inducible beta-xylanase of Thermomyces lanuginosus was investigated. Xylanase was found to be constitutively produced at a low level and induced by a number of substances. All D-pentoses as well as xylan and xylobiose were identified as inducers. The induction of xylanase by xylan was delayed by about 2 h as compared to the induction by xylose. Xylose, xylobiose, and xylan caused an induction in concentrations from 10.0 to 333.3 mg l(-1) The effect of D-lyxose, D-arabinose, D-ribose, and L-arabinose was significantly not as pronounced. D-Xylose is proposed to be a natural inducer. The time for induction was shown to be dependent on the concentration of fungal mycelium. Xylanase induction was subject to glucose repression.