Summary. Ionic channels responsible for excitation of plasmalemma and tonoplast of fresh-water Nitellopsis obtusa were studied using the voltage-clamp technique. Voltage was clamped on each separate membrane. Chloride channels were inhibited with ethacrynic acid. 1. Along with anion (chloride) channels the cation channels have been revealed in the membranes. The corresponding channels are similar in both types of membranes. 2. The cation channels are controlled by membrane voltage being activated under membrane depolarization. The channels possess activation-inactivation kinetics. For N. obtusa characteristic times of activation and inactivation are 0.1 and 0.5 sec, respectively. 3. Conductance of cation channels depends on the type of cation and the orders of conductivity decrease are the following: Rb + >K § >Cs + >Na + >Li + and Ba 2 § > Sr 2 + > Ca 2 + > Mg 2 +. Ratio of conductance for bivalent ions to that of monovalent ones decreases with the increase of normal concentrations. When the external medium contains both mono-and bivalent cations in comparable concentrations, the current is mainly determined by the latter. In natural environment of the algae such conditions are realized for Ca 2+ ions which create the bulk of the inward current through cation channels under cell excitation. That is why we term these channels "calcium" ones. 4. Ca 2 + ions entering the cytoplasm through the calcium channels located in both membranes activate the chloride channels. Ba 2+ and Mg 2+ also activate the chloride channels but to a lesser extent than Ca; § does. Characteristic inactivation time of these channels in N. obtusa is about 1 to 2 sec.