Pentavalent arsenate reductase activity was localized and characterized in vitro in the cytosolic fraction of a newly isolated bacterial strain from arsenic-contaminated sites. The bacterium was gram negative, rod-shaped, nonmotile, non-spore-forming, and noncapsulated, and the strain was identified as Pseudomonas sp. DRBS1 following biochemical and molecular approaches. The strain Pseudomonas sp. DRBS1 exhibited enzymatic machinery for reduction of arsenate(V) to arsenite(III). The suspended culture of the bacterium reduced more than 97% of As(V) (40-100 mM) to As(III) in 48 h. The growth rate and total cellular yield decreased in the presence of higher concentration of arsenate. The suspended culture repeatedly reduced 10 mM As(V) within 5 h up to five consecutive inputs. The cell-free extracts reduced 86% of 100 A mu M As(V) in 40 min. The specific activity of arsenate reductase enzyme in the presence of 100 A mu M arsenate is 6.68 A mu mol/min per milligram protein. The arsenate reductase activity is maximum at 30 A degrees C and at pH 5.2. The arsenate reductase activity increased in the presence of electron donors like citrate, glucose, and galactose and metal ions like Cd+2, Cu+2, Ca+2, and Fe+2. Selenate as an electron donor also supports the growth of strain DRBS1 and significantly increased the arsenate reduction.