Accumulation at the air-water interface of four bacterial strains with different hydrophobicities was studied. To exclude system influences, hath a static batch system and a dynamic system involving rising bubbles were used. All bacteria accumulated at the air-water interface in both systems. Generally, attachment to the air-water interface increased with bacterial surface hydrophobicity. Accumulation of two hydrophilic bacteria decreased with decreasing ionic strength, indicating that electrostatic repulsion between the negatively charged bacteria and the negatively charged air-water interface influenced the attachment. No such influence was found for hydrophobic bacteria. The calculation of DLVO interaction energies revealed that additional attractive forces were needed to explain the observed attachment. Hydrophobic interactions were calculated on the basis of contact angles and were shown to promote the accumulation. A colloid-chemical approach combining DLVO interactions and hydrophobic interactions seems to be appropriate to explain the accumulation at the air-water interface of all investigated strains.