A bacterium that reduces the soluble and toxic selenite anion to insoluble elemental red selenium (Se-0) was isolated from a laboratory bioreactor. Biochemical, morphological, and 16S rRNA gene sequence alignment identified the isolate as a Rhizobium sp. that is related to but is genetically divergent from R. radiobacter ( syn. Agrobacterium tumefaciens) or R. rubi ( syn. A. rubi). The isolate was capable of denitrification and reduced selenite to Se-0 under aerobic and denitrifying conditions. It did not reduce selenate and did not use selenite or selenate as terminal e(-) donors. Native gel electrophoresis revealed two bands, corresponding to molecular weights of similar to 100 and similar to 45 kDa, that reduced selenite. Tungsten inhibited in vivo selenite reduction, suggesting that a molybdenum- containing protein is involved in selenite reduction. This organism, or its enzymes or DNA, might be useful in bioreactors designed to remove selenite from water.