Surface electrical properties of suspensions of Escherichia coli have been studied by means of electric light scattering. From the frequency dependence of the electro-optic effect four different polarization mechanisms have been distinguished. The ionic strength and concentration dependence of the polarization mechanisms have been investigated. At particle concentrations above 3.6 X 10(7) particles/ml a long-range interparticle interaction seems to appear. The interaction mechanism is frequency dependent, with a relaxation frequency of about 1 kHz, and does not change the high-frequency electrical properties of E. coli over the studied concentration range (1.2 X 10(6)-1.2 X 10(9) particies/ml). Additionally, the approximate particle sizes have been determined electro-optically.