The interaction between cationic vesicles composed of dioctadecyldimethylammonium bromide. (DODAB) or chloride (DODAC) and Escherichia coli is described. Adsorption isotherms point to an interaction of high affinity (nonreversible adsorption) with limiting adsorption values for the amphiphiles which are compatible with vesicle adhesion to bacteria without vesicle rupture. Bacteria (10(7) bacteria/mL) in the mixtures attain a point of zero charge (pzc) at 4 x 10(-6) M DODAB as determined using microelectrophoresis. From differential turbidity measurements and optical micrographs, bacteria flocculation promoted by the vesicles is characterized and quantified. Initial flocculation rate (v(0)) for bacteria ((1.4-2.0) x 10(9) bacteria/ mL) goes through a maximum as a function of amphiphile concentration at 0.4 mM DODAB or DODAC. The results may be of importance for improving water quality in contaminated reservoirs.