Ionic selectivity of Nitella flexilis plasmalemma cation channels is studied by voltage-clamp method with consecutive replacing of cations in the bathing medium. The selectivity sequence received by measuring the ionic current reversal potentials, psi alpha is: Ba++ approximately equal to Sr++ approximately equal to Ca++ greater than Mg++ greater than Cs+ approximately equal to K+ greater than Na+ greater than Li+. An analysis of results based on the three-barrier channel model suggests that when ions of the same valency are compared, the channel selectivity is determined by specific interactions between the ion and the nearest water molecules, which is possible both in a narrow and wide pore. On the other hand, when monovalent and divalent ions are compared the effects of ions binding in the channel or near the membrane surface prevail, thus causing the channel preference for divalent cations.