Cathodic protection, using sacrificial anodes or impressed current, has been recognized for a long time as an effective way to prevent marine corrosion. Cathodic polarization leads to the formation of a protective calcareous layer on the surfaces. It is well documented that the attachment of bacteria to metal surfaces and subsequent biofilm formation changes some physical and chemical parameters at the interface and influences the corrosion process. The objective of this study was to determine whether there is a relationship between cathodic polarization and development of biofilms on surfaces exposed to both synthetic and natural seawater. Experiments were conducted on clean surfaces, biofilmed surfaces, in natural or synthetic seawater using bacterial monocultures and cocultures. In marine sediments, cathodically produced hydrogen encouraged growth of hydrogenasecontaining sulphate-reducing bacteria while in aerated seawater biofilms competed with the magnesium and calcium deposition. Both low pH induced by bacterial metabolism and exopolymers affect the deposition process and the stability of the calcareous layer.