A collection of rhizobial isolates from nodules of wild beans,<i>Phaseolus vulgaris</i> var. aborigineus, found growing in virgin lands in 17 geographically separate sites in northwest Argentina was characterized on the basis of host range, growth, hybridization to a <i>nifH</i> probe, analysis of genes coding for 16S rRNA (16S rDNA), DNA fingerprinting, and plasmid profiles. Nodules in field-collected wild bean plants were largely dominated by rhizobia carrying the 16S rDNA allele of <i>Rhizobium etli</i>. A similar prevalence of the <i>R. etli</i> allele was observed among rhizobia trapped from nearby soil. Intragroup diversity of wild bean isolates with either <i>R. etli</i>-like or <i>Rhizobium leguminosarum</i> bv. phaseoli-like alleles was generally found across northwest Argentina. The predominance of the <i>R. etli</i> allele suggests that in this center of origin of <i>P. vulgaris</i> the coevolution of <i>Rhizobium spp</i>. and primitive beans has resulted in this preferential symbiotic associationInstituto de Biotecnologia y Biologia Molecular