The filamentous fungus Trichoderma reesei forms two specific, xylan-inducible xylanases encoded by xyn1 and xyn2 to degrade the beta-1,4-D-xylan backbone of hemicelluloses. This enzyme system is formed in the presence of xylan, but not glucose. The molecular basis of the absence of xylanase I formation on glucose was the purpose of this study. Northern blotting of the xyn1 transcript as well as the use of the Escherichia coli hygromycin B phosphotransferase-encoding gene (hph) as a reporter consistently showed that the basal expression of xyn1 was affected by glucose, whereas its induction by xylan remained uninfluenced. The repression of basal xyn1 transcription is mediated by the carbon catabolite repressor protein Cre1, which in vivo binds to two of four consensus sites (5'-SYG-GRG-3') in the xyn1 promoter, which occurred in the form of an inverted repeat. T. reesei strains, bearing a xyn1::hph reporter construct, in which four nucleotides from the middle of the inverted repeat had been removed, expressed hph on glucose at a level comparable to that observed during growth on a carbon catabolite derepressing carbon source. Northern analysis of xyn1 expression in a T. reesei mutant strain (RUT C-30), which contains a truncated, non-functional cre1 gene, also confirmed basal transcription of xyn1. In this strain, xyn1 transcription was still inducible by xylose or xylan to an even higher degree than in the wild-type strain, suggesting that induction overcomes glucose repression at the level of xyn1 expression. Based on these data, we postulate that basal transcription of xyn1 is repressed by glucose and mediated by an inverted repeat of the consensus motif for Cre1-mediated carbon catabolite repression.