Batch experiments were conducted to investigate the effects of oxytetracycline on anaerobic ammonium oxidation (anammox) process. The short-and long-term effects on anammox activity were studied by measuring ammonium (NH(4)(+)), nitrite (NO(2)(-)), and nitrate (NO(3)(-)) concentrations over time. Experiments were conducted at NH(4)(+)and NO(2)(-) concentrations of 60-90 mg N/L and 60-190 mg N/L, respectively (NH(4)(+):NO(2)(-) ratio from 1:1-1:2.25), oxytetracycline concentrations of 10-100 mg/L, and biomass concentrations of 300-800 mg/L. In the short-term study, anammox activity was inhibited by all oxytetracycline concentrations studied. However, daily addition of oxytetracycline to a concentration of 5 ± 3.5 mg/L in the anammox sequencing batch reactor completely inhibited anammox activity in the fifth week. Fluorescent in situ hybridization was used to identify autotrophic ammonium oxidizing bacteria (Nitrosomonas spp., Nitrobacter spp., Nitrospira spp., Candidatus Brocadia anammoxidans, and Candidatus Kuenenia stuttgartiensis). The population of anammox culture was significantly decreased while Nitrosomonas spp. and Nitrospira spp. increased in the fifth week compared with the first week of experiment. A not-competitive model fit the anammox inhibition data at oxytetracycline concentrations of 0-100 mg/L quite well with V(max) of 0.0435 mg N/mg VSS-hr and K(i) of 54.66 mg/L.