45 root nodule samples from plants belonging to the three legume subfamilies Caesalpioideac, Mimosoideae and Papilionoideae (representing respectively 15%, 49% and 36% of the nodule samples) were collected from several places in the French Guiana's tropical forst. Moreover, 7 soil samples were collected to obtain rhizobial isolates by soil trapping on plants. Together 52 bacterial isolates were obtained and investigated by molecular analysis. Isolates from root nodules represent 9 tribes, 17 genus and 21 species, among which 3 are introduced Acacia's species, of the three legume subfamilies. The genus affiliations of the isolates were deducted from partial 16S rRNA-based phylogenetic analysis. Bacterial symbionts are predominantly (71% of the isolates) Bradyrhizobium spp. members. 29% of the isolates belongs to Rhizobium spp., distinct from described species, except one isolate belonging to Allorhizobium undicola. This is the first report of Spirotropis longifolia (tribe Sophoreae) positive nodulation status and strain identification as Rhizobium sp. The three Spirotropis longifolia's isolates group together in a cluster close to R. tropici. Partial intergenic spacer 16S-23S rRNA sequencing performed on bradyrhizobial isolates showed that all of them were divergent from the currently known Bradyrhizobium genospecies, enlighting at least nine novel potential Bradyrhizobium genospecies. Between the 37 Bradyrhizobium spp. isolates, a certain degree of host-rhizobial specificity was observed: - within the Ingeae tribe: two new potential Bradyrhizobium genospecies are composed respectively of solely Inga isolates and solely Zygia isolates - within tl Paramachaerium ormosioides) which were found both nodulated by Bradyrhizobium spp. and Rhizobium spp., the others legume species were nodulated by either bradurhizobial ou rhizobial strains. There results indicate that Frecnh Guiana is a reservoir of new rhizobial biodiversity still to be discovered. (Texte intégral)