The physiological response to ambient pH on the regulation of the production of the xylanolytic enzyme complex was investigated using two modified strains of Aspergillus nidulans. Transcriptional gene fusions were constructed between the promoters xlnA and xlnB (alkaline and acid expressed, respectively) and the Aspergillus niger goxC (encoding glucose oxidase), and A. nidulans transformants possessing single integrations at the argB locus were selected. Changing the pH from 5.5 to 4.5 or 7 after induction resulted in differential expression of the homologous and heterologous proteins. Thus, versatile industrial strains capable of differentially producing mixtures of extracellular enzymes in response to ambient pH can be produced.