This chapter reviews the different aspects of biological interactions with vesicular-arbuscular mycorrhizal (VAM) fungi. The rhizosphere effect was greatest with bacteria followed by actinomycetes and fungi. The important role of VAM fungi as components of the rhizosphere population is receiving more recognition. VAM markedly improve nodulation and nitrogen fixation by legume bacteria mainly by providing the high phosphorus requirement for the fixation process. The studies conducted so far reveal a definite synergistic interaction between free-living nitrogen-fixing bacteria and VAM in the rhizosphere with consequential improvement on plant growth. The interaction studies between VAM fungi and phosphate-solubilizing organisms have been done in pots; there is need for extending these studies to the field using cheaper forms of phosphate. The high incidence of vitamin-producing bacteria and actinomycetes in the rhizosphere and the greater amount of vitamins they produce suggest that vitamins synthesized by these organisms may contribute significantly to mycorrhizal development.