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Plasmids obtained from four field isolates of Rhizobium leguminosarum were visualized following electrophoresis on agarose gels. The relative mobilities of the bands observed corresponded to plasmids with molecular weights of about 100 × 10⁶ and greater. Each field isolate examined had a different pattern of plasmids.

Lysates from R. leguminosarum strain 300 normally produced three plasmid bands, although in some preparations two much larger plasmids were also visible. The smallest plasmid band seen in strain 300 probably contains two co-migrating plasmids, because in one derivative of strain 300 it was replaced by a doublet, presumably as a result of the presence of a small deletion in one of the co-migrating plasmids. No apparent symbiotic defects were associated with the presence of this deletion. However, a non-nodulating derivative of strain 300 (strain 6015) was found to have suffered a deletion …