Clostridium barkeri incorporates selenium into the enzyme nicotinic acid hydroxylase as shown by copurification using numerous protein separation techniques. Nicotinic acid hydroxylase, like many flavoproteins, possesses NADPH oxidase activity. The activity when nicotinic acid serves as the substrate is more sensitive than the NADPH oxidase to reduction and alkylation, treatments that frequently modify selenium compounds. This suggests that the selenium is associated with the nicotininc acid-reactive site on the enzyme.