The corrosion behavior of carbon steel (CS) was investigated in 3.5% NaCl solution in the presence and absence of Spirulina platensis (SP) bacteria. For this purpose, the weight loss measurements, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization techniques were used. The counts of SP were monitored with UV-VIS spectrophotometer, optical microscope and the dissolved oxygen (DO) was determined in all solutions. The morphology of the electrodes surfaces were characterized with SEM images. The results show that, SP decreased the corrosion rate and the current density values of CS; the corrosion potential was shifted to nobler values, the polarization resistance values were increased. All parameters reveal that the SP effectively protected CS against corrosion and reduced its corrosion rate in 3.5% NaCl solution. (C) 2011 Elsevier B.V. All rights reserved.