The coloured ciliate Blepharisma japonicum changes swimming velocity (positive photokinesis) and elongates its body in response to a prolonged illumination. We have recently proposed that alterations in the phosphorylation level of the ciliate phosducin (Pdc) may be involved in light-induced cell elongation, which in turn affects the interaction of βγ-dimer of G-proteins (Gβγ) with β-tubulin and subsequent cytoskeletal remodelling. The cellular mechanism that governs the photokinetic effect in this ciliate has not been elucidated. In the present study, we utilise real-time PCR to demonstrate that the levels of ciliate Pdc mRNA are significantly reduced in Pdc-RNAi-treated cells compared to cells fed with bacteria carrying the empty vector (control cells). Using western immunoblotting, we confirmed that these cells treated with Pdc-RNAi expressed a substantially lower level of the Pdc protein. The assay also revealed that in ciliates treated with Pdc-RNAi and exposed to light, the cytosolic level of Gβ (~36 kDa) was reduced, whereas the level of Gβ localized to the membrane (~32 kDa) was increased compared to control cells. In addition, behavioural analysis of the cells indicated a substantial reduction of photokinesis. The findings in this study provide additional characterization of the functional properties of the ciliate Pdc protein and we discuss a likely role for this phosphoprotein in the photokinetic phenomenon of the ciliate protist Blepharisma.