The present investigation was designed to assess the effect of growth-promoting activities of Rhizobium sp^_ a diazotroph, on plant growth and a number biochemical parameter including in vivo nitrogenase activity in the root rhizosphere of Zea mays L. The Rhizobium in this study was isolated from the root nodules of Vigna mungo L. and it was applied in three way, viz., seed soaking, seedling soaking, and basal treatment. Plants of Zea mays L. were raised in earthenware pots containing soil from paddy field. The results showed significant enhancement in various biochemical parameters (different fractions of leaf pigments (chlorophyll-a and chlorophyll-b, and carotenoids), total soluble sugars, phosphate buffer soluble proteins, total free amino acids, total phenol) and activities of different enzymes, viz., in vivo nitrate reductase activity and in vitro catalase activity in the leaves and in vivo nitrogenase (N2-ase) activity in the roots’ rhizosphere; and growth parameters such as lengths of root  and coleoptiles and their fresh and dry weights and leaf area also increased. The over all observations revealed that use of Rhizobium sp from the root nodules of Vigna mungo L. improved the photosynthetic efficiency, nitrogen-metabolizing capacity and plant growth to significant extent and it may be used as a source of potent bio-fertilizer for maize crop.