We have subcloned two plasmids, pWS-6BP1 and pWS-6BP2, from the 8.5 kb DNA fragment of plasmid pJB22-B6. The plasmid of pWS-6BP1, with a 5.8 kb DNA fragment, can complement the exopolysaccharide-deficient mutant NA06 and NA12 of Rhizobium huakuii strain 107, but the plasmid pWS-6BP2, with a 2.6 kb DNA fragment can only complement NA12. The effective nodules formed by the wild type strain 107 (Rh107) contain lots of bacteriods; the ineffective nodules formed by the mutant NA06 and NA12 contain few bacteroids. Although the nodule formed by Exo(+) transconjugants are ineffective, the bacteria can be successfully released into the cells and form lots of bacteroids. The Rh107 produces acidic exopolysaccharides (EPS) of high molecular weight (HMW) in large amount with only small amount of low molecular weight (LMW) forms. EPS secreted by mutants NA06 and NA12 are just the other way round with LMW in higher amount than HMW ones. They also produce an EPS with molecular weight in between. The EPS secreted by the Exo(+) transconjugants is more like the EPS of Rh107 than that of mutants, although their component and structure are still different from that of Rh107.