Corrosion inhibition of SAE 1018 steel by pure-culture biofilms of Pseudomonas fragi and Escherichia coli DH5 alpha has been evaluated in complex Luria-Bertani medium, seawater-mimicking medium, and modified Baar’s medium at 30 degrees C. In batch cultures, both bacteria inhibited corrosion three to six fold compared to sterile controls, and the corrosion was comparable to that observed in anaerobic sterile media. To corroborate this result : a continuous reactor and electrochemical impedance spectroscopy were used to show that both P. fragi K and E. coli DH5 alpha decreased the corrosion rate by 4- to 40-fold as compared to sterile controls; this matched the decrease in corrosion found with sterile medium in the absence of oxygen and with E. coli DH5 alpha grown anaerobically. In addition, the requirement for live respiring cells was demonstrated by the increase in the corrosion rate that was observed upon killing the P. fragi K biofilm in continuous cultures, and it was shown that fermentation products do not cause an increase in corrosion. Hence, pure-culture biofilms inhibit corrosion of SAE 1018 steel by depleting oxygen at the metal surface.