A static mercury electrode was used for measurement of double-layer charge displacement signals caused by living plant cells of the unicellular marine alga Dunaliella tertiolecta. By scanning the electrode potential a point is reached where the charge density of a plant cell compensates the electrode charge density. The experimentally determined values of surface charges for unicellular marine alga Dunaliella tertiolecta cells are -0.63 and -0.75 muC/cm(2) in 0.1 m NaCl and 1 m NaCl solutions, respectively.