The effects of predation on nitrification performance and microbial community during bioaugmentation were investigated. Although most of the nitrification ability of the seed source was lost in the seeded reactors, bioaugmentation significantly enhanced the activity and community of the nitrifiers. The ammonium uptake rate (AUR) increased from 2.59 to 15.25 mg NH(4)(+)-N/L h and 2.88 to 13.36 mg NH(4)(+)-N/L h, and the nitrite uptake rate (NUR) increased from 0.80 to 4.02 mg NO(2)(-)-N/L h and 0.76 to 4.34 mg NO(2)(-)N/L h for the reactors with and without protozoa inhibition, respectively. The population of nitrifiers increased, and the dominant nitrite oxidizing bacteria (NOB) transferred from Nitrospira to Nitrobacter. Predation had an evident influence on the microbial community of nitrifiers, especially the K-strategist, which was more vulnerable to predation than r-strategist during bioaugmentation due to its low growth rate. However, predation did not have a significant effect on the nitrification performance. (C) 2011 Elsevier Ltd. All rights reserved.