The load of some pesticides into water environments is seriously concerned for its adverse effects on non-target organisms. In this study, we investigated the effects of some neonicotinoid insecticides on diatom (the primary producer in aquatic ecosystems) phototaxis. Neonicotinoids had both positive and negative effects on the phototaxis, depending on the organisms and the chemical species investigated. Imidacloprid promoted Nitzschia sp. phototaxis up to 160 mg L^-1 (the highest concentration in this study), while thiacloprid decreased the relative number of cells exhibiting positive phototaxis in Nitzschia sp. and Mayamaea pseudoterrestris by up to approximately 29 % and 70 %, respectively. However, the decrease in phototaxis is unlikely at the detected concentrations in the environment. Our results suggest that phototaxis is useful as an endpoint for ecotoxicological impact assessment. The method based on counting the number of cells can evaluate the ecotoxicological impact of some neonicotinoids on diatom phototaxis and may be applied to growth inhibition evaluation in an environmental situation where light is irradiated unevenly.