The molecular mechanism of montmorillonite flocculation by bacterial polysaccharides was investigated, with special emphasis on the effect of carboxylic charges in the macromolecules on the mechanisms of interaction with the clay surface. An indirect way to quantify the energy of interaction was used, by comparing the flocculation ability of variously acidic polysaccharides. Data on tensile strength of aggregates in diluted suspension were collected by timed size measurements in the domain 0.1-600 microm, using laser diffraction. The flow behavior of settled aggregates was studied by rheology measurements. Flocculation of colloidal clay suspension by polysaccharides requires cancelling of the electrostatic repulsions by salts, which allows approach of clay surfaces close enough to be bridged by adsorbing macromolecules. The amount of acidic charges of the polysaccharides, and especially their location in the molecular structure, governs the bridging mechanism and the resulting tensile strength of the aggregates. The exposure of carboxylate groups located on side chains strongly promotes flocculation. In turn, charges located on the backbone of the polysaccharide are less accessible to interaction, and the flocculation ability of such polysaccharides is lowered. Measurements at different pH indicate that adsorption of acidic polysaccharides occurs via electrostatic interactions on the amphoteric edge surface of clay platelets, whereas neutral polysaccharides rather adsorb via weak interactions. Increased tensile strength in diluted aggregates due to strong surface interactions results in proportionally increased viscosity of the concentrated aggregates.