Each of four wild-type isolates of Rhizobium meliloti carried single moderately large (100–200 megadaltons) plasmids in addition to their megaplasmids. Such moderately large plasmids from these strains had either considerable or little sequence homology to one another. Sequence homology was not correlated to the geographical origin of the strains. In one selected strain where this was examined, loss of the moderately large plasmid did not reduce the nodulating or nitrogen-fixing ability of the strain in laboratory experiments. In this strain, the moderately large plasmid was found to be selftransmissible by mating between R. meliloti.