The effect of changes in zeta potential on the flocculation of Klebsiella suspensions over the pH range from 4 to 9 was studied by using polyacrylamide（PAM）as flocculating agents.The results indicated that the zeta potential of Klebsiella cells depended on isoelectrical point，and was positive below pH 2.2 and negative in the pH range 4—9.In addition，the zeta potentials of non-ionic polyacrylamide（nPAM）and anionic polyacrylamide（aPAM）were negative as the pH varied in all cases，which led to electrostatic repulsion between negatively charged polymer and negatively charged Klebsiella surface.However，cationic polyacrylamide（cPAM）was strongly pH dependent，and the zeta potential of cPAM varied from positive to negative values with isoelectrical point at pH 8.From flocculation tests，it was seen that cPAM showed good flocculation separation of Klebsiella cells with flocculation ratio(FR) of 94.5% at pH 7.The superior performance of cPAM in comparison with aPAM and nPAM could be attributed to the charge-neutralization with Klebsiella cells.Through the comparison among complexation flocculation of PAM and aluminium sulphate，when aluminium sulphate was present，it was seen that the FR of nPAM was considerably greater than the case when no electrolyte was present with the maximum FR 95.1% occurring at pH 4.The results demonstrated that the flocculating role of charge neutralization gave priority over polymer bridging and this was crucial to optimizing flocculants and flocculating process of Klebsiella suspensions.