Cross-linked POIY-γ-glutamic acid (C-L γ-PGA) markedly purified polluted water collected from rivers and ponds by flocculation and precipitation. This effect of C-L γ-PGA occasionally required pretreatment with polyaluminum chloride (PAC). Components of polluted water in rivers or ponds are generally thought to be clay minerals, microorganisms and chemical compounds. In this study, the flocculating activities of C-L γ-PGA against suspensions of bentonite, diatomaceous earth, Escherichia coli and Mycrocystis aeruginosa, and against solutions of crystal violet and bisphenol A were investigated. The mode of action of C-L γ-PGA is thought to be based on electrostatic interaction between flocculants, C-L γ-PGA and PAC, and the surface of polluted water components, which may lead to neutralization of the zeta-potential of those components.