Xyr1 (xylanase regulator 1) of the ascomycete Hypocrea jecorina (anamorph Trichoderma reesei) was recently demonstrated to play an essential role in the transcriptional regulation of the xyn1 (xylanase 1-encoding) gene expression. Consequently, this study reports on the deletion of the xyr1 gene from the H. jecorina genome. Comparative studies of the growth behavior of the different mutant strains (deleted and retransformed xyr1) grown on various carbon sources pointed to the strongly reduced ability of the xyr1 deletion strain to utilize D-xylose and xylan. Transcriptional analysis of the xyl1 (D-xylose reductase 1-encoding) gene as well as measurements of corresponding enzymatic activities gave evidence that Xyr1 takes part in the control of the fungal D-xylose pathway, in particular in the regulation of D-xylose reductase. It could be demonstrated that the uptake of D-xylose into the fungal cell is uninfluenced in the Deltaxyr1 strain. Furthermore, transcriptional regulation of the major hydrolytic enzyme-encoding genes xyn1 and xyn2 (xylanases 1 and 2), cbh1 and cbh2 (cellobiohydrolases 1 and 2), and egl1 (endoglucanase 1) is strictly dependent on Xyr1. Regulation of the respective genes via Xyr1 is not affected by the substances mediating induction (xylose, xylobiose, and sophorose) and is indispensable for all modes of gene expression (basal, derepressed, and induced). Moreover, Xyr1, it was revealed, activated transcriptional regulation of inducer-providing enzymes such as beta-xylosidase BXLI and beta-glucosidase BGLI but was not shown to be involved in the regulation of BGLII.