The protozoan ciliate Paramecium bursaria exhibits membrane hyperpolarization in response to photostimulation, accompanied with an increased swimming speed. The external addition of cyclic nucleotide phosphodiesterase (PDE) inhibitors, either theophylline (1,3-dimethylxanthine) or 3-isobutyl-1-methylxanthin (IBMX), increased in both amplitudes of the membrane hyperpolarization and the increase in swimming speed. Moreover, the addition of membrane permeable cyclic nucleotide analogs, either 8-bromo-adenosine 3′,5′-cyclic monophosphate (Br-cAMP) or 8-Br-guanosine 3′,5′-cyclic monophosphate (Br-cGMP), increased these amplitudes. On the other hand, the addition of l-cis-diltiazem, known to block the conductance of cyclic nucleotide–gated channels, partially decreased both amplitudes of the membrane hyperpolarization and the increase in swimming speed. An enzyme immunoassay of cellular cyclic nucleotide contents showed that photostimulation induced a rapid increase in adenosine 3′,5′-cyclic monophosphate (cAMP), but little increase in guanosine 3′,5′-cyclic monophosphate (cGMP), raising the possibility that a rapid increase in cAMP mediates the light-induced hyperpolarization in P. bursaria.