A soil microcosm system was used to evaluate the impact of a genetically modified (GM) Rhizobium meliloti strain on development and function of arbuscular mycorrhiza in alfalfa plants (Medicago sativa L.). There was no indication that this GM Rhizobium strain, which had an enhanced nodulation competitiveness ability, interfered with mycorrhiza formation by Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe. Indeed, inoculation with the GM Rhizobium strain greatly increased the number of mycorrhizal entry points in the alfalfa root system in comparison to the wild-type strain. Mycorrhizal development and quality of nodulation increased with time and coincided with increased biomass of and nutrient (N, P) uptake by the host plant. The establishment of the symbiotic interactions also induced changes in root morphology; in particular, the degree of branching increased and the number of lateral roots was greater in plants inoculated with the GM Rhizobium strain. These results demonstrate that the GM Rhizobium strain does affect adversely the performance of arbuscular mycorrhizal symbiosis, a biosafety model system based on a functional rhizosphere.