The stability of stainless steel 304 in the presence of aerobic and anaerobic bacteria was investigated in a continuous flow system using electrochemical impedance spectroscopy and scanning electron microscopy (SEM) examinations. The results show that the open circuit potential (OCP) of stainless steel was ennobled (shifted in the electropositive direction) by about +150 mV in the presence of the aerobic bacteria. The observed change of OCP in the electropositive direction can be explained by an increase in the rate of the cathodic reaction. Also the presence of an aerobic biofilm led to a decrease in the polarization resistance of stainless steel is not only due to the growth of micropits, as shown from SEM micrographs, but also due to thinning of the passive film. Ln the presence of only Pseudomonas fragi, the electrochemical impedance response showed a capacitive behavior with RP on the order of 500 k Omega. The addition of anaerobic sulfate reducing bacteria (SRB) to the test medium decreased RP to 12 k Omega due to an increase in localized corrosion, as indicated by SEM examination.