Six monoclonal antibodies were produced to whole cells of Shewanella colwelliana (Aco1 to Aco6) and two (Aco22 to Aco23) to purified exopolysaccharide (EPS). Aco1, -4 to -6, -22, and -23 bound to both the cell surface and the purified EPS, while Aco2 and -3 bound to cells only. The EPS of S. colwelliana was antigenically unique from those of nine other species of marine bacteria that were tested. Mapping studies revealed that all of the EPS-specific monoclonal antibodies bound to the same epitope. This EPS epitope was sensitive to cleavage of ester bonds, but neither pyruvate, acetate, nor terminal nonreducing sugars were required for antigenicity. When S. colwelliana was grown on rich media, most of its EPS was loosely associated with the cell surface.