The loss of fertilizer-N in submerged paddy soil was investigated with ¹⁵N-labelled ammonium sulfate in special designed pots. The gaseous composition of the atmosphere in the upper-space above the water in the sealed pot controlled by transmitting or N₂ gas during the experiment period (4-6 weeks after transplanting).The N through nitrification-denitrification was estimated by deducting the ¹⁵NH3 volatilized from the deficit in the balance sheet of ¹⁵N-labelled fertilizer-N The results obtained are discribed as follows: 1. In the neutral paddy soil investigated, nitrification-denitrification was the most important pathway of the losses of fertilizer-N. 2. The loss of fertilizer-N in the treatment of transmitting N₂ gas through the upper-space in the sealed pots approximated to that of transmitting air, implying that. nitrification-denitrification taken place in the oxidized layer on the soil surface and the reduced layer under it may not be the sole mechanism of nitrification-denitrification losses of fertilizer-N applied as basal dressing in soil. 3. The losses of fertilizer-N in the pots with rice were markedly lower than that without rice. The competition of the absorption of ammonium by rice plant with nitrification may be the reason of the decrease of N losses. 4. In the treatment of transmitting N₂ gas through the upper-space in the pots without rice, there was still a significant loss of fertilizer-N expelling the soil ari by transmitting N₂ gas through the solum before flooding had little effect on the reduction of N losses. Therefore, it is necessary to further investigate whether any other oxides might be able to act as an electron acceptor in the oxidation of ammonium in submerged paddy soil, resulting in a subsequent denitrification loss.