N-Phenyl-1-naphthylamine (P(1)NA) and N-Phenyl-2-naphthylamine (P(2)NA) are both widely used as antioxidant and plant secondary metabolites. In this study, growth, esterase, photosynthetic activity and cell membrane integrity were used as biomarkers to compare biotoxicity of P(1)NA and P(2)NA on Microcystis aeruginosa. According to the results, a dose-response relationship was observed only between P(1)NA concentrations and growth inhibition. The EC50 (48 h) of P(1)NA calculated from growth inhibition was 16.62 mu M, while that of P(2)NA was not detected. When the esterase and photosynthetic activity were applied to evaluate the biotoxicity, it was found that a concentration of 20 mu M P(1)NA, P(2)NA caused reduction of esterase activity and Fv/Fm of M. aeruginosa to 22.2 and 3.3%, 97.5 and 92.1%, respectively, after 48 h exposure. The percentage of membrane-damaged cells was increased as P(1)NA exposure concentration increased, but that was not detected when exposure to P(2)NA. The difference substituted position in the molecular structure of P(1)NA and P(2)NA leads to different toxicological properties and only P(1)NA was found highly toxic to M. aeruginosa. The toxicity is due to that only P(1)NA can be biotransformed to 1,4-naphthoquinone, which could induce overproduction of intracellular ROS as well as result in oxidative damage and growth inhibition of test organism.(C) 2017 Elsevier Ltd. All rights reserved.