The effect of carbon sources on the production of beta-xylanase by Thermomyces lanuginosus TISTR 3465 was investigated. Xylan showed the highest inductive effect on the enzyme formation whereas xylobiose and xylooligosaccharides resulted in lesser effect. beta-Xylanase was also produced at low level with xylose as well as other sugars tested. Xylan concentration at 15 g L(-1) gave the maximum inductive effect on beta-xylanase formation, whereas xylooligosaccharides and xylose were effective at a lower concentration of 2.5 g L(-1). High concentrations of these sugars significantly repressed the enzyme formation. Crude enzyme from the supernatants, without and with other sugars produced a single xylanase band on non-denaturing PAGE gels. However, an intense xylanase activity band was observed from the supernatant of media with xylan, xylobiose and xylooligosaccharides as the carbon sources. An intense protein band of 24.9 kDa from the culture filtrate of xylan medium was observed. Xylan increased beta-xylanase production by the fungus 16-fold when it was added to the xylose medium after cultivation for 3 days. In contrast, addition of xylose to the xylan medium decreased beta-xylanase production 3-fold. A distinct appearance and disappearance of a 24.9 kDa protein and the activity band coincided with an increase and decrease of xylanase activity, respectively. This indicated the synthesis of xylanase by this strain was most induced by xylan. Moreover, the level of xylanase induction has no related to amino acid sequence of the enzyme.