The broad host range plasmidp SUP 104 carrying the streptomycin sensitivity gene - S12, was m obilized from Escherichiaco1i S17-1 to Rhizobium meliloti 2 011, R.l er.uminosarum 897 and two Brad vrhizobium.iaponicum serogroup 123 strains. Transconjugants were purified from the two Rhizobium strains. The plasmid pSUP104-S12 failed to replicate in the B..Iaponicum serogroup 123 strains and no transconjugants were isolated. The plasmid pSUP104-S12 induced a streptomycin sensitive phenotype in R.me 1 i 1 oti 2011 and IU1 inosarum which were previously streptomycin resistant. The selection for streptomycin resistant revertants carrying the pSUP104-S12 plasmid caused insertion of IS elements directly into the S12 gene. Two IS elements were isolated, one ISRm2 is a 1.25 Kb IS element isolated from R.me 1 i loti 2011, the other ISR11 is a 2.7 Kb IS element isolated fron R.leguminosarum 897. Both IS elements caused deletions of the pSUP104-S12 plasmid in a Rec A- E.coli strain. Hybridization analysis using 32P labelled pSUP104-S12::ISRm2 as a probe with EcoRl digests of total chromosomal DNA of various Rhizobium and B..iaponicum serogroup 123 strains revealed that ISRm2 was particular to R.me1 i 1oti strains. By the same method ISR11 was found in all the Rhizobium strains tested and in most of the B..iaponicum serogroup 123 strains. It would be possible to use ISR 1 1 as a pr o b e in DNA fingerprinting to identify a B,.iaponicum serogroup 123 inoculum from an indigenous B..iaponicum serogroup 123 in field trials of a serogroup 123 inoculum. Both IS elements were implicated in DNA rearrangements in Rhizobium strains.