The rate of N2 fixation was evaluated in four experiments using Crowley silt loam soil incubated under flooded conditions. When a flooded soil not planted to rice was incubated under light for a period of 2 yr, the N2 fixation rate, as determined by 15N tracer, was 57 μg/g/yr, whereas the N2 fixation rate measured by the increase in total N content was 61 μg/g/yr. Dinitrogen fixation rate, measured by the acetylene reduction method, was increased considerably when N-poor rice straw was added to the soil. In undisturbed soil cores taken from a rice field, 15N fixation was relatively low in relation to the plant's need, occurring only under light and only in the surface layer of soil. Aquatic weeds growing in these soil cores contained appreciable amounts of 15N. Measurement of N2 fixation by the acetylene reduction method in plexiglass enclosures in the field also showed a low fixation of N2.