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In lowland areas rhizobia can fix nitrogen in legume association and colonize the rice plants in rotation. Recent studies have shown that rhizobia can promote plant growth by colonizing rice roots, stems and leaves. The production of phytohormones, mainly indole acetic acid (IAA) is possibly the main mechanism of growth promotion of rice by rhizobia. The objectives of this study were to check the responsiveness of different rice cultivars to inoculate with rhizobia; to study the colonization pattern of rice plants by rhizobia and legumes; and to genetically evaluate the biosynthesis of auxin by rhizobia. Several experiments were conducted in the greenhouse and in the laboratory with rice plants inoculated with different rhizobia. The rice cultivars tested IRGA424 proved to be more responsive to inoculation with rhizobia. Rhizobial strains marked with the gusA gene confirmed the colonization of rice. The rhizobia tested are able to produce IAA and there is evidence that the biosynthesis occurs by the route of indole-3-acetonitrile (IAN).