Aspergillus niger secretes a number of enzymes, including proteases, into its culture fluid. The regulation of the two major acidic extracellular proteases, pepA and pepB, was investigated using Northern analyses. Our data suggest that the regulation of pepA and pepB expression occurs predominantly at the level of mRNA content and that, while they are regulated in a similar manner, differences are also clear in their expression. Both genes were found to be under complex regulatory control. The expression of the two genes could be turned off by the presence of good nitrogen or carbon sources in the media, and external protein sources did not induce expression of either gene under conditions of carbon and nitrogen repression. The pH of the medium also played a major role in their regulation as the expression of both genes was completely turned off under alkaline conditions, even when grown in media lacking good nitrogen and carbon sources but containing proteins. We isolated clones containing 5' non-coding sequences of the pepA gene from a lambda genomic library with a pepA specific probe. Analysis and comparison of the promoter sequences of the pepA and pepB genes revealed that both contain several putative AREA- and CREA-binding sites and they also share an 18-bp-long sequence which is 83% identical in these two genes.