Microbiologically influenced corrosion (MIG) of mild steel samples were studied in two different bacterial strains, Pseudomonas fluorescens and Desulphovibrio vulgaris, isolated from cutting oil emulsions by potentiostatic polarization technique. The relationship between the corrosion behaviour and presence of biofilm on mild steel samples were determined. Bacteria were mixed with corrosion products hindering the formation of a protective layer. In both media, anodic depolarization was observed in the presence of ferric iron reducing bacteria. Anodic depoloarization was achieved by the bacterial reduction of ferric to ferrous compounds. In the absence of the bacteria, both anode and cathode were polarized.