Immunoblotting of isolated cell membrane fractions from ciliates Blepharisma japonicum and Stentor coeruleus with a polyclonal antibody raised against rhodopsin revealed one strong protein band of about 36 kDa, thought to correspond to protozoan rhodopsin. Inspection of both ciliates labeled with the same antibody using a confocal microscope confirmed the immunoblotting result and demonstrated the presence of these rhodopsin-like molecules localized within the cell membrane area. Immunoblot analysis of the ciliate membrane fractions resolved by two-dimensional gel electrophoresis identified two distinct 36 kDa spots at pIs of 4.5 and 7.0 for Blepharisma, and three spots at pIs of 4.4, 5.0 and 6.0 for Stentor, indicating a possible mixture of phosphorylated rhodopsin species in these cells. The obtained results suggest that both Blepharisma and the related ciliate Stentor contain within the cell membrane the rhodopsin-like proteins, which may be involved as receptor molecules in the sensory transduction pathway mediating motile photoresponses in these protists as in other species of lower eukaryota.