In a Ðeld investigation, nitrogenase activity and nitrogen-Ðxing bacteria associated with rhizosphere of rice cultivars with varying N absorption efficiencies were examined at three N-fertiliser regimes. A signiÐcant di †erence among the cultivars with regard to the nitrogenase activity of rhizosphere bacteria was observed. Under no or low (30 kg N ha~1) fertiliser application, the cultivar with high boot leaf-N and high apparent N recovery had a higher nitrogenase activity. At 60 kg N ha~1 fertiliser application, though the nitrogenase activity was stimulated, the di †erences with regard to nitrogenase activity among the cultivars were not signiÐcant. Microbiological analysis indicated that the cultivar with high N absorption efficiency harboured higher populations of nitrogen-Ðxing Azospirillum sp, Azotobacter sp and anaerobic bacteria. Under 60 kg N ha~1 fertiliser application, the rhizosphere of both cultivars, which differed in their N use pattern, harboured almost identical population densities of nitrogen-Ðxing bacteria. Interestingly, the anaerobic nitrogen-Ðxing bacteria were greatly stimulated following the application of fertiliser. Notwithstanding the e †ect of N fertiliser management on Ðxation, N absorption efficiency of rice N 2 crop and the rhizosphere associated nitrogenase activity and nitrogen-Ðxing bacteria were found to be interrelated. Further investigation are required to evaluate the exact relationships including higher number of cultivars.