Two field experiments were conducted in June 1997 and 1998 to study the effect of dual inoculation with Rhizobium as symbiotic N₂-fixer and Azospirillum lipoferum as asymbiotic N₂-fixer, biogas manure and mineral fertilizer on nodulation, N₂-ase activity and yield of guar plants [Cyamopsis tetragonoloba ( L.) Taub.]                 Investigation results showed that, guar seeds application with dual inoculation of Rhizobium and Azospirillum lipoferum and provided with half dose of biogas manure (20 kg N/fed), gave the highest values of bacterial densities in soil rhizosphere, CO₂ evolution, N₂-ase activity, nodulation and guar seed contents of  total carbohydrates and guar gum.                 Dual inoculation combined with half dose of inorganic nitrogen or half dose of organic nitrogen (20 kg N/fed) increased all values than combination with full dose (40 kg N/fed) of each inorganic nitrogen or organic nitrogen fertilizers.                 Also, individual Rhizobium inoculation supplied with 40 kg N/fed organic nitrogen or inorganic nitrogen increased the values of all treatments than individual inoculation with dual inoculation received the full dose of organic or inorganic nitrogen.