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A plasmid-cured derivative of Rhizobium loti strain NZP2037, strain PN4010, was significantly more effective in N2 fixation and showed an improved capacity to compete with other root nodule bacteria for nodulation of Lotus pedunculatus, relative to that of strain NZP2037. Reintroduction of the plasmid pRlo2037 into PN4010 resulted in a return to the NZP2037 level of symbiotic effectiveness and competitiveness. The enhanced effectiveness and competitiveness of PN4010 for L. pedunculatus was associated with its increased capacity to form nodules. Strain PN4010 did not differ from NZP2037 in its growth characteristics in pure culture, in its resistance to L. pedunculatus root flavolan or in its ability to multiply in the L. pedunculatus rhizosphere in the presence of a competing strain of R. loti. An examination of 35 R. loti strains (including 25 field isolates) revealed two strains (NZP2014 and NZP2042) that did not contain a single large indigenous plasmid; both were significantly more effective in N2 fixation with L. pedunculatus than was NZP2037 or two other Nod+ Fix+ plasmid-containing strains of R. loti. Transfer of pRlo2037 into NZP2014 and NZP2042 reduced their effectiveness for L. pedunculatus.