A field inoculation experiment using two Bradyrhizobium sp. strains was set up in Madagascar to test the growth response of Acacia mangium and to follow up the survival of inoculant strains using molecular tools. Three months after inoculation, one of the inoculant strains, AUST13c, exhibited a marked growth-promoting effect with a shoot height about 40% higher than that of the uninoculated control plants or TEL8-inoculated plants. The positive effect on tree growth initially observed with AUST13c was no more significant 6 months after transfer to the field and disappeared completely at 13 and 19 months. Analyses of nodule bacterial rRNA by PCR/RFLP displayed an early contamination of the different inoculation treatments by AUST13c 3 months after inoculation, spreading to almost all nodules of the trial 6 months later. This work clearly demonstrated that the progressive reduction of the positive effect of AUST13c inoculation on tree growth after field transplanting was not due to a progressive disappearance of this introduced strain but, on the contrary, was related to the widening spreading of AUST13c in all the plots. This was attributed to a higher competitiveness and effectiveness of AUST13c over the local strains and TEL8.