In this paper, we report the identification of replication/partition regions of plasmid p42a and p42b of Rhizobium etli CFN42. Sequence analysis reveals that both replication/partition regions belong to the repABC family. Phylogenetic analysis of all the complete repABC replication/partition regions reported to date, shows that repABC plasmids coexisting in the same strain arose most likely by lateral transfer instead of by duplication followed by divergence. A model explaining how new incompatibility groups originate, is proposed.