One of the major problems with the production of biotechnologically valuable proteins has been the purification of the product. For Escherichia coli and Saccharomyces cerevisiae, there are several techniques for the purification of intracellular proteins, but these are time consuming and often result in poor yields. Purification can be considerably facilitated, if the product is secreted from the host cell. In the work presented, we have constructed an expression vector (pSGNH2) for the secretion of protein disulfide isomerase (PDI; EC 5.3.4.1) from Aspergillus niger, in which the retention signal His-Asp-Glu-Leu (H-D-E-L) was modified to Ala-Leu-Glu-Gln (A-L-E-Q) via the polymerase chain reaction (PCR) method. The PDI gene was placed under the control of the A. oryzae alpha-amylase promoter. This expression vector was transformed into A. niger NRRL3, resulting in PDI secretion into the medium. The catalytic activity of overexpressed PDI from A. niger was indistinguishable from that of PDI isolated from bovine liver. With further strain improvement and optimization of culture conditions, it could be possible to raise the PDI production to the bioprocessing scale.