The filamentous fungi Aspergillus niger and Hypocrea jecorina (Trichoderma reesei) have been the subject of many studies investigating the mechanism of transcriptional regulation of hemicellulase- and cellulase-encoding genes. The transcriptional regulator XlnR that was initially identified in A. niger as the transcriptional regulator of xylanase-encoding genes controls the transcription of about 20-30 genes encoding hemicellulases and cellulases. The orthologous xyr1 (xylanase regulator 1-encoding) gene product of H. jecorina has a similar function as XlnR, although at points, the mechanisms seems to be different. Specifically in H. jecorina, the interaction of Xyr1 and the co-regulators Ace1 and Ace2 in the regulation of transcription of xylanases and cellulases has been studied. This paper describes the similarities and differences in the transcriptional regulation of expression of hemicellulases and cellulases in A. niger and H. jecorina.