Stentor coeruleus, an unicellular protozoan ciliate, shows both negative phototaxis and step-up photophobic responses with respect to visible light. Thus, the organism swims along the propagation direction of light, while at the same time avoiding brighter light spots. The aim of our study was to elucidate the mechanism(s) of these photosensory transduction processes of. Stentor in terms of structure and function of the primary photoreceptor proteins. Stentor coeruleus was mass cultured in a 10-liter bottle, as described previously (1). Microscopic observation of swimming pattern and photophobic responses was carried out under a Zeiss microscope or a stereo microscope. Analysis of the photoresponse behavior of Stentor was made by means of a video system equipped with an infrared sensitive Newvicon tube. The actinic light beam was introduced from underneath the cuvette containing Stentor cells. Monochromatic light was produced by means of interference filters. Electrophysiological experiments were carried out by means of internal micro glass electrodes, as described elsewhere(2). The photoreceptor proteins of Stentor were isolated and purified by combined procedures based on ammonium sulfate precipitation, sucrose density gradient centrifugation, and isoelectric focussing in a sucrose density gradient containing ampholine(pH 3-10) at 500V. These procedures are available elsewhere (1). Molecular weights of the proteins were determined by the SDS polyacrylamide gel electrophoresis. The phobic response of Stentor consists of a stop response after a delay of about 0.2 sec. The organisms turn sideway and swim awim away in a new direction. Phobic response can be elicited by both spatial steps in light intensity and by light flashes. The action spectrum coincides with the absorption spectrum of Stentor as well as that of the isolated photoreceptor. The chromophore of the latter was identified as a covalently linked hypericin-peptide. Negative phototactic orientation of Stentor shows an action spectoum with maximum at 610㎚, consistent with the photophobic action spectrum and the absorption spectrum of the photoreceptor. The mechanism of phototaxis involves detection of the light intensity impinging on the side facing the light source daring orientation in lateral light, which is compared with that detected by the photoreceptors on the opposite side. This asymmetric detection produces a signal which causes a reorientation by the ciliary beat in Stentor(3). The ionophores, gramicidin and A 23187, do not specifically inhibit photoreponses of Stentor. However, the protonophorous uncouplers such as TPMP^+ and FCCP specifically impair the photophobic and phototactic responses. Since these agents block light-induced membrane potential changes at the same concentrations, we propose that the mechanism for the photosensory transduction in Stentor utilizes a light-induced proton gradient generated by the proton conducting network of the membrane-bound photoreceptor as a result of excitation by light. Thus, we have been able to observe the light-induced pH gradient in whole cell suspension of Stentor, reminiscent of the light-induced proton release by Halobacter halobium. In order to elucidate the proposed mechanism of photosensory transduction in terms of function of the Stentor photoreceptor, we have isolated the photoreceptor which consists of four major subunit proteins, each linked to the common. chromophores. The molecular weights of these subunits are 13,000, 16,000, 65,000 and 130,000 daltons. Spectroscopic characterization of the photoreceptor proteins by fluorescence, fluorescence polarization, nanosecond and picosecond time-resolved spectra and circular dichroism has been carried out, and results are consistent with the proposed mechanism that the photoreceptor triggers the generation of proton gradient across the Stentor plasma membrane upon excitation by light. The riliary motor apparatus is in turn coupled with the light-induc