We have cloned an Aspergillus nidulans gene (prtA) encoding an alkaline protease (Alp) by probing an A. nidulans library with a fragment amplified from an Aspergillus oryzae Alp-encoding gene. The nucleotide (nt) sequence of prtA was determined. The structure of prtA is similar to that of the A. oryzae Alp-encoding gene. The prtA gene is composed of four exons which are separated by three introns of 59, 57 and 54 nt. The deduced amino acid sequence of the prtA product shows a high degree of similarity to proteases from A. oryzae, A. fumigatus and A. flavus. Southern blot analysis suggests that only one copy of this gene is found in the genome of A. nidulans. The extracellular proteases of A. nidulans are regulated by nitrogen, carbon and sulfur metabolite repression. The prtA RNA levels were analysed under different nutrient conditions. No prtA transcript was detected in mycelium grown in medium containing glucose, NH4+ and sulfate. However, prtA transcript levels were high in mycelia transferred to medium lacking a nitrogen, carbon or sulfur source.