Anaerobic ammonium oxidation with an anode as the electron acceptor was realized in a dual-chamber microbial electrolysis cell (MEC). Nitrate was the main product that accounted for approximately 95% of ammonium consumed, but nitrite was also detectable. Using 16S ribosomal RNA analysis, we found that the microbial community attached to the electrode was dominated by Nitrosomonas europaea (40.3%) and the genus Empedobacter (34.7%), but no anammox bacteria were detected. Nitrosomonas europaea was shown to be necessary with an inhibition assay using allylthiourea. Certain soluble metabolites were found to have an important effect on the current production. These results show that there are many ways to oxidize ammonium biologically.