Superoxide dismutase (SOD, EC 1.15.1.1) from nitrogen-fixing Azotobacter chroococcum was purified and identified as being similar to the manganese SOD of other procaryotes. The enzyme was relatively thermostable and insensitive to cyanide. A molecular weight of approximately 33 000 was estimated. Superoxide dismutase was found to be cytoplasmic (not bound to cell membranes) in A. chroococcum, but some enzyme was released by sonication of membrane vesicles.