The zeta -potential of cells of Escherichia coli 163 and Streptococcus faecalis 134 in aqueous salt solutions has been studied by micro-electrophoresis. It has been established that, with a rise in the concentration of electrolytes in the water, the value of the zeta -potential depends on the concentration of the salts and falls more intensively for E. coli cells than for Str. faecalis cells. Possible reasons for the phenomena observed are discussed.