Heterotrophic nitrogen fixation, as influenced by water treatment, cellulose, and ammonium sulfate was investigated in four tropical, Indian rice soils; the 15N tracer technique was used for the first time. Despite high salinity and acidity, appreciable nitrogen fixation occurred in the acid sulfate and the saline soils under both flooded and nonflooded conditions. Soil submergence accelerated nitrogen fixation in all soils. Addition of cellulose to both nonflooded and flooded soils enhanced nitrogen fixation. In flooded alluvial and laterite soils, nitrogen fixation decreased with increasing concentrations of ammonium sulfate; under nonflooded conditions, this inhibition was more pronounced, particularly in laterite soil. Nitrogen fixation was completely inhibited by ammonium sulfate in the acid sulfate and the saline soils.