The photodynamic effects of methylene blue (MB) on wild-type and mutant strains of Paramecium Were studied. From measurements of survival and cell motility under the continuous application of light in the presence of MB, the mutant strains remained alive for about three times longer than the wild-type strain. Although the resting potential of the mutant cells was similar to that of wild-type cells, the continuous photodynamic action shifted the membrane potentials of the mutant and wild-type cells to a depolarized level and a hyperpolarized level, respectively, from that before light application. Under voltage clamping, the mutant cells reduced not only the outward current elicited by the photodynamic action but also the outward tail current elicited by the preceding pulse of hyperpolarization. We conclude that the mutant strain is defective in the activation of Ca(2+)-dependent K+ channels. This defect might cause a reduction in the Ca2+ influx because of the suppression of the membrane hyperpolarization, which results in the elongation of survival time under the photodynamic action.