This study examined the biodegradation potential of three wastewater micropollutants (triclosan, bisphenol A, and ibuprofen) by Nitrosomonas europaea and mixed ammonia-oxidizing bacteria in nitrifying activated sludge. N. europaea could degrade triclosan and bisphenol A, but not ibuprofen. The degradation was observed only in the absence of allylthiourea (an inhibitor for ammonia monooxygenase (AMO)), suggesting that AMO might be responsible for triclosan and bisphenol A degradation. Competitive inhibition among ammonia, triclosan, and bisphenol A was observed. Inactivation of N. europaea was observed after degrading a mixture of triclosan and bisphenol A. The inactivation might be due to product toxicity and/or antimicrobial effect of triclosan; however, the causes of the inactivation were not determined. Regardless of the presence of the AMO inhibitor, three micropollutants were degraded by two different nitrified activated sludge samples. The results suggested that both ammonia-oxidizing bacteria and heterotrophic microorganisms in the activated sludge can degrade triclosan and bisphenol A. On the other hand, ibuprofen was more likely degraded by heterotrophic microorganisms in the activated sludge.