Nitrogen fixing abilities of six Dalbergia sissoo seed sources, namely, Sibsagar (Assam), Tulsipur-Gonda, Chiriyapur, Hissar (Haryana), Simblewala (Jammu and Kashmir) and Kankai (Nepal), from the entire distribution range of this species in India and Nepal were studied at the Forest Research Institute, Dehra Dun (32 deg 20'40"N latitude, 77 deg 52'12" longitude), India from 1997 to 1999. Significant differences in nodulation behaviour, nitrogenase activity and soil nitrogen accretion were observed between different seed sources. Maximum nodule biomass and nitrogen fixation were recorded in Gonda seed source followed by Nepal, Chiriyapur, Jammu, Hissar and minimum in Assam. A positive correlation was noticed between nodule number and its biomass with nitrogen fixation ability. The differences were discernible and pronounced only during the second year of growth. These are important to exploit the genetic variability among different seed sources of D. sissoo in amelioration of degraded sites.