The performance, microbial enzymatic activity and microbial community of a sequencing batch reactor (SBR) were investigated under long-term exposure to cerium dioxide nanoparticles (CeO2 NPs). The COD removal kept a stable value at 0-5 mg/L CeO2 NPs and then decreased at 10-60 mg/L CeO2 NPs. The NH4+-N removal had no obvious changes at 0-30 mg/L CeO2 NPs, and a minor decrease appeared at 60 mg/L CeO2 NPs. Compared to 0 mg/L CeO2 NPs, the phosphorus removal showed a decrease at 2 mg/L CeO2 NPs and slightly increased at 5-60 mg/L CeO2 NPs. The nitrogen and phosphorus removal rates had similar variation trends to the microbial enzymatic activities. The variations of reactive oxygen species (ROS) and lactate dehydrogenase (LDH) indicated that high CeO2 NPs concentration could result in the biotoxicity to activated sludge. The presence of CeO2 NPs had obvious effect on the microbial richness and diversity of activated sludge. (C) 2016 Elsevier Ltd. All rights reserved.