Adhesion of three marine bacterial strains, i.e. Marinobacter hydrocarbonoclasticus, Psychrobacter sp. and Halomonas pacifica with different cell surface hydrophobicities was measured on glass in a stagnation point flow chamber. Prior to bacterial adhesion, the glass surface was conditioned for 1 h with natural seawater collected at different seasons in order to determine the effect of seawater composition on the conditioning film and bacterial adhesion to it. The presence of a conditioning film was demonstrated by an increase in water contact angle from 15° on bare glass to 50° on the conditioned glass, concurrent with an increase in the amount of adsorbed organic carbon and nitrogen, as measured by X-ray photoelectron spectroscopy. Multiple linear regression analysis on initial deposition rates, with as explanatory variables the temperature, salinity, pH and concentration of dissolved organic carbon (DOC) of the seawater at the time of collection, showed that the concentration of DOC was most strongly associated with the initial deposition rates of the three strains. Initial deposition rates of the two most hydrophilic strains to a conditioning film, increased with the concentration of DOC in the seawater, whereas the initial deposition rate of the most hydrophobic strain decreased with an increasing concentration of DOC.