The influence on cell hydrophobicity of differential extension with ionic strength of lipopolysaccharide molecules (LPS), which exist as charged and uncharged polymers at the surface of the Gram-negative bacterium Pseudomonas aeruginosa (PA01), has been investigated. Attenuated total reflection infrared (ATR-IR) spectral absorptions from a single layer of cells adsorbed to ZnSe increased in intensity with increasing NaCl concentration up to 0.1 mat L-1. Dynamic contact angle measurements (Wilhelmy plate tensiometry) made with a ZnSe plate having an adsorbed cell layer and the adherence of the cells to hexadecane suggest that PA01 cells were most hydrophobic in contact with 0.1 mol NaCl solutions. These data indicate a charge screening induced compression of the charged LPS polymers decreasing the cell-surface approach distance and increasing the cell hydrophobicity due to the greater surface predominance of the uncharged LPS polymers. Interestingly, adsorbed cell layers in 0.3 tool L-1 NaCl had a lower IR absorption intensity, and PA01 cells suspended in 0.3 mol L were found to be more hydrophilic, indicating that other factors influence the cell-surface approach distance and hydrophobicity. The examination of cell electrophoretic mobility variation with NaCl concentration suggests that the compression of charged polysaccharides increases the polysaccharide charge density and may also reduce the flow of liquid through the polysaccharide layer affecting the effective potential at the interface, the cell hydrophobicity, and the cell-surface approach distance.