The growth response of Phaseolus vulgaris to dual inoculation with arbuscular mycorrhizal (AM) fungi and rhizobia was studied in a low-fertility tropical soil in Madagascar. Two isolates of AM fungi identified as Acaulospora sp. and Glomus sp., respectively, along with a cocktail of ten Rhizobium spp. strains were used to conduct a greenhouse experiment in a fully randomized block design with two factors. The Phaseolus vulgaris seedlings received one of the following inoculation treatments: no inoculation, separate inoculation with each of the three microbial symbionts (the two AM fungal isolates and the rhizobia), and co-inoculation with each of the two AM fungal isolates and the mix of rhizobium strains. The results showed an additive effect of co-infection by AM fungi and rhizobia on plant growth and on the total N content of the plants, along with a synergistic effect on the total P content, the number of nodules and the mycorrhizal rate of the plants. Dual symbiosis with native strains contributes to the success of legumes, especially in harsh environments and low-fertility tropical soils.