Nitrogen fixing efficiency of sodium azide-resistant strains of Rhizobium leguminosarum bv. trifolii was studied in symbiosis with berseem clover plants in chillum jars. Rate of respiration and glutamine synthetase activity were tested in cultured cells and nodules, respectively. It was observed that shoot dry weight and percentage shoot nitrogen were maximum in plants inoculated with strains resistant to 15 μg ml−1 sodium azide. Rate of respiration in cultured cells was lowest in strains resistant to 15 μg ml−1 sodium azide and highest in strains resistant to 5 μg ml−1 sodium azide. A negative correlation was observed between rate of respiration (in cultured cells) and shoot dry weight of host plants. Glutamine synthetase activity was maximum in nodule extracts of host plants inoculated with strains resistant to 5 and 10 μg ml−1 sodium azide, whereas it was minimum for strains resistant to 15 μg ml−1 sodium azide. Hence, resistance to low doses (15 μg ml−1) of sodium azide, together with lower respiratory and glutamine synthetase activities, could be used as a potential method for isolating the symbiotically effective strains of Rh. leguminosarum bv. trifolii.