Reactive oxygen species (ROS) generated by harmful algal blooms (HABs) exert detrimental effects on aquaculture systems. Fish gill cells deteriorate upon exposure to HABs, suggesting that internally generated ROS in HABs influences the external environment. Therefore, we investigated the internal and external changes in ROS concentrations during growth using fluorescence staining of four representative HABs: Alexandrium affine, Chattonella marina, Karenia mikimotoi, and Margalefidinium polykrikoides. The concentrations of H2O2 and O2 - produced by A. affine were low; H2O2 from M. polykrikoides was primarily detected internally throughout the experiments, and O2 - was not detected. High H2O2 and O2 - concentrations were observed in K. mikimotoi during the death phase, with weak external O2 - concentrations. Regarding C. marina, which produces large amounts of ROS, H2O2 was observed internally during the exponential phase, whereas weak O2 - concentrations were measured externally in the stationary phases. Collectively, our results highlight that ROS concentrations and internal/external distributions are functions of HABs and growth stage. These differences indicate the potential allelopathic mechanisms of proliferating HABs and suggest a possible impact of ROS on aquaculture organ...