Photocatalytic inactivation of algae, Anabaena, Microcystis, and Melosira, was performed with TiO2-coated pyrex glass beads under the illumination of UV light (370 nm wavelength). After being irradiated with UV light in the presence of the TiO2-coated pyrex glass beads, Anabaena and Microcystis, known as typical cyanobacteria, lost their photosynthetic activity, and the string of Anabaena cells and the colonies of Microcystis cells were completely separated into individual spherical ones. In the case of Melosira, which is a typical diatom, however, somewhat lower photocatalytic inactivation efficiency was obtained, which was believed to be due to the presence of the inorganic siliceous wall surrounding the cells of Melosira.