The effect of various carbohydrates on the production of inducible β-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.