We amplified a 1.1-kb fragment of DNA from genomic DNA of Aspergillus oryzae using the polymerase-chain reaction (PCR) and primers, the sequences of which correspond to the consensus sequences coding for two thioredoxin domains of protein disulfide isomerase (PDI). A genomic DNA clone of A, oryzae was isolated by hybridization with the amplified DNA fragment, Southern hybridization analysis revealed that the gene encoding PDI (pdiA) was present in a 5.0-kb XhoI region of the A, oryzae chromosome as a single copy. pdiA contains three putative introns and encodes 515 amino acid residues. The deduced amino acid sequence elf A. oryzae PDI exhibits a high degree of homology (51%) with Humicola insolens PDI, while it exhibits less than 20% homology with those of yeast, plants, and mammals. The amino acid sequence contains an N-terminal signal peptide-like sequence, the C-terminal putative endoplasmic reticulum (ER) retention signal, HDEL, and two putative active site sequences (WCGHCK) of PDI, When the cloned pdiA was expressed under the control of the amyB promoter in an A. oryzae transformant, the PDI activity in the microsomal fraction was about 10-fold higher than that in a microsomal fraction of the recipient strain, indicating that tile cloned gene encodes functional PDI.