Adsorption isotherms of quaternized polyvinylpyridine on the surface of Escherichia coli bacterial cells were performed using a spectrophotometer. Results show that about 1 mg of polymer can adsorb per m(2) of bacterial surface. Electrophoretic mobility measurements of the cells for various quantities of adsorbed polymer indicate that the charge of the cells can be inverted by the cationic polyelectrolyte. Combining the adsorption isotherm and the electrophoretic mobility measurements, we argue that the polymer chains are adsorbed in a very flat configuration on the surface and that they form a strongly entangled network with a mesh size which decreases to a molecular size when the quantity of adsorbed polymer increases to the plateau value of the adsorption isotherm.