Shewanella colwelliana (strain D) is a periphytic estuarine bacterium that forms biofilms beneficial to oyster set. Our study examined whether these and other films concentrated two hydrophobic, organophosphate pesticides, Abate and malathion, that are detected in Chesapeake Bay oyster waters. Both biofilms and purified exopolysaccharide of S. colwelliana did not adsorb more of the Abate or malathion than could be accounted for by adsorption to control surfaces. Similar results were obtained by using Deleya marina, Hyphomonas MHS3, and autochthonous biofilms. Conversely, decapsulated S. colwelliana D cells, prepared in the laboratory, bioconcentrated Abate. Significantly, the S. colwelliana D biofilms exposed to Abate did not inhibit the settlement and metamorphosis of Crassostrea gigas larvae.