Biobarriers remove, via precipitation, the metalloid selenite (SeO (3) (-2) ) from groundwater; a process that involves the biological reduction of soluble SeO (3) (-2) to insoluble elemental red selenium (Se-0). The enzymes associated with this reduction process are poorly understood. In Rhizobium selenitireducens at least two enzymes are potentially involved; one, a nitrite reductase reduces SeO (3) (-2) to Se-0 but another protein may also be involved which is investigated in this study. Proteins from R. selenitireducens cells were precipitated with ammonium sulfate and run on native electrophoresis gels. When these gels were incubated with NADH and SeO (3) (-2) a band of precipitated Se-0 developed signifying the presence of a SeO (3) (-2) reducing protein. Bands were cut from the gel and analyzed for peptides via LCMSMS. The amino acid sequences associated with the bands indicated the presence of an NADH:flavin oxidoreductase that resembles YP_001326930 from Sinorhizobium medicae. The protein is part of a protein family termed old-yellow-enzymes (OYE) that contain a flavin binding domain. OYE enzymes are often involved in protecting cells from oxidative stress and, due in part to an active site that has a highly accessible binding pocket, are generally active on a wide range of substrates. This report is the first of an OYE enzyme being involved in SeO (3) (-2) reduction.