Abstract: Nodulation potential, nitrogen fixation efficiency (nitrogenase activity) and biomass yield in response of Leucaena leucocephala and Sesbania sesban to inoculation with auxotrophic mutants of fast growing Rhizobium strains was explored in short-term field trials. All the strains formed nodules and fixed nitrogen in both hosts with some relative differences. The diversity of rhizobia that form symbioses with the roots of both hosts, an economically important leguminous tree species, was examined by inoculating seedling root zones with samples of auxotrophic mutants of rhizobia derived from both mutagens, acridine and ascorbic acid. Nitrogen fixation, total nitrogen accumulation, and plant growth varied significantly among both hosts seedlings inoculated with the representative isolates. All auxotrophic mutants derived from the parental strain FFAMU-8, stimulated chlorophyll (a) formation in Sesbania sesban, relative to the negative control. Although, three of auxotrophic mutants, asc2-FFAMU-8, asc3-FFAMU-8 and asc4-FFAMU-8 affected to significantly increase chlorophyll (b) formation than their negative and positive control plants. Both auxotrophic mutants, asc3-FFAMU-8 and asc3-ARCG-10 affected to significantly greater below ground biomass components in Leucaena leucocephala than their in the positive control plants. In addition, some of auxotrophic mutants derived from the strain HRT-27 affect to significantly increase woody, aerial and root biomass over the positive control of Sesbania sesban. Some of auxotrophic mutants appeared reliable ranking for nodule development/plant biomass among both hosts. Many of significant correlations were obtained among both tree legumes between nodulation, nitrogen fixation parameters with plant growth criterion. The results support the use of efficient rhizobial strains to inoculate woody legumes for improving plant survival and biomass development.