Background and AimsInoculation of legumes with effective N2-fixing rhizobia is a common practice to improve farming profitability and sustainability. To succeed, inoculant rhizobia must overcome competition for nodulation by resident soil rhizobia that fix N2ineffectively. In Kenya, wherePhaseolus vulgaris(common bean) is inoculated with highly effectiveRhizobium tropiciCIAT899 from Colombia, response to inoculation is low, possibly due to competition from ineffective resident soil rhizobia. Here, we evaluate the competitiveness of CIAT899 against diverse rhizobia isolated from cultivated KenyanP. vulgaris.MethodsThe ability of 28 KenyanP. vulgarisstrains to nodulate this host when co-inoculated with CIAT899 was assessed. Rhizosphere competence of a subset of strains and the ability of seed inoculated CIAT899 to nodulateP. vulgariswhen sown into soil with pre-existing populations of rhizobia was analyzed.ResultsCompetitiveness varied widely, with only 27% of the test strains more competitive than CIAT899 at nodulatingP. vulgaris. While competitiveness did not correlate with symbiotic effectiveness, five strains were competitive against CIAT899 and symbiotically effective. In contrast, rhizosphere competence strongly correlated with competitiveness. Soil rhizobia had a position-dependent numerical advantage, outcompeting seed-inoculated CIAT899 for nodulation ofP. vulgaris,unless the resident strain was poorly competitive.ConclusionSuboptimally effective rhizobia can outcompete CIAT899 for nodulation ofP. vulgaris. If these strains are widespread in Kenyan soils, they may largely explain the poor response to inoculation. The five competitive and effective strains characterized here are candidates for inoculant development and may prove better adapted to Kenyan conditions than CIAT899.