Homologues to Sinorhizobium meliloti insertion sequences ISRm3 and ISRm4 were found within Rhizobium leguminosarum bv. viciae field populations. Similarly, homologous sequences to S. meliloti ISRm4 were found in S. fredii strains. Based on the polymerase chain reaction, we cloned and partially sequenced some of these putative insertion sequence elements. DNA sequence comparisons indicate that S. meliloti and R. leguminosarum ISRm3-type elements are closely related whereas the ISRm4 homologues show distinct relationships. In addition, specific primers for PCR recognition of ISRm3 and ISRm4 elements in rhizobia species and S. meliloti ISRm6 were designed.