A thin film of titanium dioxide (TiO₂, anatase crystalline form) coated on a glass plate inactivated Lactobacillus casei PL-1 phages suspended in a buffer solution, when the reaction mixture was illuminated with a black-light lamp (maximum wave length, 365 nm) and shaken gently. When the reaction mixture was not illuminated, the TiO₂ film exhibited no phageinactivating activity. TiO₂ was not photoexcited by white (fluorescent)-light lamp. The degree of phage inactivation was directly proportional to the surface area of the TiO₂ film. The phage inactivation approximately followed first-order reaction kinetics. The phage inactivation by photoexcited TiO₂ film was inhibited by superoxide dismutase and D-mannitol, and accelerated by hydrogen peroxide, indicating that the phage inactivation is due to the active oxygen species generated on the surface of TiO₂ film under the black-light illumination. Electron microscopic observation of the negatively-stained preparation revealed that about 43% of the phages treated with photoexcited TiO₂ film were converted into ghost-particles with empty heads.