The effects of isolated marine Pseudoalteromonas sp.on the microstructure and corrosion behaviour of 2205 duplex stainless steel were investigated using electrochemical and surface analysis methods.Electrochemical studies demonstrated a negative shift in corrosion potential and an increase in corrosion current density in the presence of bacteria.EDS results showed a high concentration of chloride ions in the biofilm structure and a decrease in Cr content beneath the biofilm layer and near cracks.These results could lead to localised corrosion on metal surfaces.FESEM images illustrated the process of bacterial attachment on the metal surfaces at different exposure times.