Water coproduced with petroleum may contain sulfides and organic constituents that give the water an aquatic toxicity preventing surface discharge. A simulated sour produced water and actual field samples of produced water were successfully biotreated with mixed cultures of Thiobacillus denitrificans and floc-forming heterotrophs. Complete removal of benzene, toluene, phenol, acetic acid, sulfides, and Microtox toxicity was achieved. These results indicate that a reactor system as simple in concept as a specialized activated sludge system can be used to treat produced water with these mixed contaminants, allowing surface discharge of the water for reuse.