An anammox assay involving a ¹⁵N tracer and gas chromatography-mass spectrometry revealed that the potential anammox activity accounted for 1 to 5% of total N₂ production in a ravine paddy field, Japan. Among four 4-cm-deep layers, the top layer showed the highest activity. Clone libraries showed that the DNA in the top layer contained sequences related to those of Candidatus 'Brocadia fulgida', Ca. 'B. anammoxidans', and Ca. 'Kuenenia stuttgartiensis'. These results suggest that a specific population of anammox bacteria was present in paddy soils, although a small part of dinitrogen gas was emitted from the soil via anammox.