Shewanella colwelliana, a marine bacterium which grows in association with oysters, has been found to produce an unusual extracellular polymer which mediates irreversible adhesion of the bacterium to surfaces. Production of the polymer was enhanced when the bacterium was grown on marine agar, overlaid with a dialysis membrane, and seeded with a liquid suspension of the cells. Purified exopolymer was shown to contain Ca, S, P and Si (40–45%), lipid (10%), carbohydrate (15–35%) and protein (<5%). Glucose was the predominant component of the carbohydrate moiety. Uronic acids were not detected. The exopolymer forms a loose capsule that surrounds the bacterium, demonstrated by light and electron microscopy.