It is widely known that the phototransduction process in vertebrate photoreceptor cells are mediated by cGMP-gated ionic channels. The recent electrophysiological study showed that the channels might play a key role in phototransduction process which leads to the photophobic behavior in the ciliate. The basic biophysical and pharmaceutical properties of cGMP-gated channels in Stentor indicate high similarity to those in vertebrate photoreceptor cells.