The relationship of photosynthesis and photocatalytic inhibition of algal growth (Chroococcus sp.) by TiO2 was investigated by an ATP assay under ultraviolet (UV) and fluorescent light (FL) irradiations. The adverse effects of what on Chroococcus sp. growth in the TiO2+FL+UV run are larger than those in the TiO2+UV run. The difference is considered to be caused by oxygen generated by Chroococcus sp. under FL irradiation. A rate equation for the relationship of light irradiation intensity to rate of decrease in the Chroococcus sp. population on the basis of the Langmuir-Hinshelwood model has been proposed. The experimental data are in good agreement with the proposed model. In this study, the optimum FL intensity is 0.51 mW/cm(2).