Background and purpose: Many investigations of the influence of temperature, ultrasound, electromagnetic waves, heavy metals, and ionic solutions on the cell membrane potentials have confirmed that the membrane potential is very sensitive to physical and chemical effect of the surroundings. The purpose of this work was to investigate the influence of a dispergent, as the possible pollutant of aquatic ecosystems, upon the membrane potential of algal cells. Material and methods: Experiments were performed on freshwater algae Nitella belonging to the Caharaceae family. The potential were measured using the microelectrode technique. A microelectrode was inserted into the cell vacuole and the reference electrode was immersed into the basal solution containing equal parts of 0.1 mM KCl and 1.0 mM NaCl. This solution was contaminated with various concentrations of a dispergent (from 0.0001% to 10%). Results: The observed changes of resting membrane potentials showed that the dispergent had caused the depolarisation of the membrane, proportionally with concentration if the dispergent was successively increased. If the basal solution was contaminated directly with dispergent concentration of 0.01%, thr hyperpolarization of the membrane occured. Control cells lived longer than experimental ones that were contamined with 0.01% and 10% concentrations of dispergent. Control cells had more oscillations than experimental cells, but the observed difference is not statistically significant. Conclusion: The observed changes of resting membrane potentials showed the dispergent caused the depolarisation of the membrane, proportionally with its concentration.