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This study was committed to examining the symbiotic effectiveness difference between wild and mutant rhizobial isolates, and at the same time to appreciate the enhancement effect of mutagenesis on tolerance of common bean (Phaseolus vulgaris L.) nodulating rhizobia to extreme salinity and pH conditions. A total of 50 isolates were obtained from soil samples of three weredas in Hararghe lowlands and mid altitudes using the host trap method and were presumptively identified as rhizobia. After mutagenesis, a total of 8 mutants were selected based on their ability to survive at extreme salt and pH conditions. All of the mutants were found to be symbiotically effective. Among the observed rhizobium isolates, HUCR (3D, 3A), HUCR 2D, and HUCRM 2D showed the highest symbiotic effectiveness. Only the mutant isolates HUCRM2D (which tolerated 12% NaCl, pH4 and pH12), HUCRM5C (which tolerated 12% NaCl and pH 4), HUCRM3B (which tolerated 12% NaCl) and HUCRM9C (which tolerated 11% NaCl) were growing successfully at the indicated extreme conditions. Thus, based on their symbiotic effectiveness and tolerance to extreme environmental conditions, these wild and mutant isolates were recommended to be used as candidates for the future development of rhizobial inoculants of common bean grown under saline, extreme temperature, and pH conditions.