The effect of the singlet oxygen quencher crocetin on the photomotile responses and photodynamic killing of the ciliated protozoan Blepharisma japonicum was investigated. The percentage of cells surviving photodynamic killing is higher in the presence of the water-soluble carotenoid derivative than in control samples. However, no significant difference is observed in the photobehavior of the control and crocetin-containing samples. These results indicate that blepharismin-sensitized photodynamic reactions occur in Blepharisma, but do not play any meaningful role in initiating photosensory processes.