Nitrogen balance studies based on Kjeldahl data are reported for several crops of rice (Oryza sativa) grown in continuously flooded soil in pots under greenhouse conditions in the Philippines. Flooded soil planted to rice had a statistically significant positive nitrogen balance. The positive nitrogen balance was found to be the result of photo-trophic and heterotrophic nitrogen-fixing agents. Adding phosphorus and iron to the pots of flooded soil planted to rice significantly increased the positive nitrogen balance. Blue-green algal (cyanobacterial) inoculation of the soils fertilized with these nutrients did not have a statistically significant effect, although azolla inoculation in combination with these nutrients did significantly increase the positive nitrogen balance. Heterotrophic fixation depended upon the presence of rice plants. It is not possible to determine from these nitrogen balance data if the rice plant enhances fixation or simultaneously enhances fixation and reduces nitrogen losses. These experiments offer an explanation for the natural nitrogen fertility of flooded rice fields and indicate that rice plants can significantly improve the nitrogen economy of flooded soils.