Résumé : Biofouling induced corrosion of stainless steels grades UNS S31603 and UNS S31254 was investigated at three sites off Singapore for 30months. No corrosion was observed on grade UNS S31254, while the propensity for corrosion by shellfish of UNS S31603 is ranked as oyster>>barnacles>>green mussels. Oysters caused extensive localised corrosion, penetrating 2mm thick plates with 12months with tracks of corrosion several centimetres long. Shallow crevice corrosion was observed under dead barnacles, with an explanation presented for why corrosion is more severe under dead barnacles than live ones. Green mussels did not cause any corrosion.