Anaerobic ammonium-oxidizing (anammox) bacteria have been recognized as an important sink for fixed nitrogen and are detected in many natural environments. However, their presence in terrestrial ecosystems has long been overlooked, and their contribution to the nitrogen cycling in natural and agricultural soils is currently unknown. Here we describe the enrichment and characterization of anammox bacteria from a nitrogen-loaded peat soil. After 8 months of incubation with the natural surface water of the sampling site and increasing ammonium and nitrite concentrations, anammox cells constituted 40 to 50% of the enrichment culture. The two dominant anammox phylotypes were affiliated with “Candidatus Jettenia asiatica” and “Candidatus Brocadia fulgida.” The enrichment culture converted NH4+ and NO2− to N2 with the previously reported stoichiometry (1:1.27) and had a maximum specific anaerobic ammonium oxidation rate of 0.94 mmol NH4+·g (dry weight)−1·h−1 at pH 7.1 and 32°C. The diagnostic anammox-specific lipids were detected at a concentration of 650 ng·g (dry weight)−1, and pentyl--ladderane was the most abundant ladderane lipid.