The hydrophobic interaction between the surfaces hydrophobized by the adsorption of STAC in a STAC solution (system I) and the interaction between the surfaces hydrophobized by the reaction with OTS in water (system II) were investigated using an atomic force microscope, and their difference was compared. Clear differences of the interaction between systems I and II were found with respect to the force curves and the morphology of the surfactants adsorbed on the surfaces. It is postulated that the hydrophobic interaction for system II is attributable to the cavity bridging between the surfaces, but that for system I it is originated from a different mechanism. It is also implied that the existence of the surfactants in the bulk will play an important role for the hydrophobic interaction, (C) 1999 Academic Press.