Three different classes of thiol-oxidoreductases that facilitate the formation of protein disulfide bonds have been identified. They are the Ero1 and SOX/ALR family members in eukaryotic cells, and the DsbB family members in prokaryotic cells. These enzymes transfer oxidizing potential to the proteins PDI or DsbA, which are responsible for directly introducing disulfide bonds into substrate proteins during oxidative protein folding in eukaryotes and prokaryotes, respectively. A comparison of the recent X-ray crystal structure of Ero1 with the previously solved structure of the SOX/ALR family member Erv2 reveals that, despite a lack of primary sequence homology between Ero1 and Erv2, the core catalytic domains of these two proteins share a remarkable structural similarity. Our search of the DsbB protein sequence for features found in the Ero1 and Erv2 structures leads us to propose that, in a fascinating example of structural convergence, the catalytic core of this integral membrane protein may resemble the soluble catalytic domain of Ero1 and Erv2. Our analysis of DsbB also identified two new groups of DsbB proteins that, based on sequence homology, may also possess a catalytic core similar in structure to the catalytic domains of Ero1 and Erv2.