A sequence of sorption of different bacterial types has been observed both on glass slides and electron microscope grids immersed in seawater for periods of up to 24 h. A comparison of morphological groups of bacteria initially attracted to a surface with those subsequently adhering firmly to the surface suggests a selective irreversible sorption of small rods. Firm adhesion of bacteria in the time periods considered does not indicate pili or holdfast structures. The ability of bacteria to produce extracellular polymeric fibrils may be important in such selective sorption.