The common bean (Phaseolus vulgaris L.) occupies more than 120,000ha in Iran but information on the genetic structure and nitrogen fixation potential of the rhizobia nodulating this legume crop is scarce. Fifty-three bacterial isolates were sampled from root nodules of common bean cultivated in 12 different provinces of Iran. The genetic diversity of the isolates was analysed by PCR–RFLP of the 16S–23S rDNA internally transcribed spacer (ITS) region. At the similarity of 64%, common bean rhizobia were clustered into 10 groups, demonstrating a high degree of genetic variability among the isolates. Significant differences existed in the symbiotic potential of the isolates examined under greenhouse conditions. Ten isolates showed promising symbiotic characteristics in terms of shoot dry weight and symbiotic efficiency, and selected isolates revealed increased plant growth and yield under field conditions.