The occurrence of facultative symbiotrophic N2-fixing associations in three rice soils of India is reported. Considerable variation in N2-fixing efficiency of these associations was noticed among the soil types studied. Associations from rice straw-amended alluvial soil under both flooded and non-flooded conditions exhibited higher N.-fixing efficiency than those from unamended soils of both water regimes. Despite high salinity and acidity an acid sulphate soil harboured N.-fixing symbiotrophic organisms with appreciable efficiency. Application of rice straw to the soil under both flooded and non-flooded conditions stimulated N2-fixation in alluvial, laterite and acid sulphate saline soils. These observations suggested the significant contribution of these associations to the nitrogen economy of different soil types.