Examination of ciliate Blepharisma japonicum whole cell lysates with an antibody against phosphoserine and in vivo labeling of cells with radioactive phosphate revealed that the photophobic response in the ciliate is accompanied by a rapid dephosphorylation of a 28 kDa protein and an enhanced phosphorylation of a 46 kDa protein. Analysis with antibodies raised against rat phosducin or human phosducin-like proteins, identified one major protein of a molecular weight of 28 kDa, and two protein bands of 40 kDa and 93 kDa. While the identified ciliate phosducin is phosphorylated in a light-dependent manner, both phosducin-like proteins exhibit no detectable dependence of phosphorylation upon illumination. An immunoprecipitation assay also showed that the ciliate phosducin is indeed phosphorylated on a serine residue and exists in a phosphorylated form in darkness and that its dephosphorylation occurs in light. Immunocytochemical experiments showed that protozoan phosducin and phosducin-like proteins are localized almost uniformly within the cytoplasm of cells adapted to darkness. Cell exposure to light caused a pronounced displacement of the cell phosducin to the vicinity of the plasma membrane; however, no translocation of phosducin-like proteins was observed upon cell illumination. The obtained results are the first demonstration of the presence and morphological localization of a possible phosducin and phosducin-like proteins in ciliate protists. Phosducin and phosducin-like proteins were found to bind and sequester the betagamma-subunits of G-proteins with implications for regulation of G-protein-mediated signaling pathways in various eukaryotic cells. The findings presented in this study suggest that the identified phosphoproteins in photosensitive Blepharisma japonicum may also participate in the regulation of the efficiency of sensory transduction, resulting in the motile photophobic response in this cell.