Nitrifier enriched culture (NEC) cultivated in laboratory and nitrifying activated sludge taken from STPs were used to biodegrade estrone (E1) and 17α-ethinylestradiol (EE2). The correlation between ammonia oxidation activity and estrogens biodegradation activity was investigated by varying initial ammonia concentration and inhibiting the activity of ammonia oxidizing bacteria (AOB) with allylthiourea. Estrogens biodegradation activity in NEC was significantly dependent on the activity of AOB, and the nitration of estrogens as well as biodegradation by heterotrophs was minimal. The co-metabolism of estrogens by AOB was identified. In NAS, estrogens biodegradation still happened with ATU inhibition, implying that AOB and heterotrophs cooperated to biodegrade E1 and EE2. However, the co-metabolism of estrogens by AOB remained the most efficient biodegradation pathway in NAS.