A new mutant ofParamecium letraureliahas been isolated with a profound defect in the regulation of membrane potential. This mutant,restless, hyperpolarizes as a potassium electrode below 8 mM external K+whereas wild-type cells can maintain a constant resting cell potential independent of low external K+concentration,restlessdies in solutions of low K+concentration in which wild-type can survive indefinitely.restlessis not allelic to mutations that affect the depolarization-dependent Ca2+current, the Ca2+-activated K+current, and the Ca2+-activated Na+current. The results suggest thatrestlessis a new class of mutant affecting a K+conductance hitherto not characterized genetically inParamecium.